Dockerfile reference

Dockerfile reference

Docker can build images automatically by reading the instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession.

This page describes the commands you can use in a Dockerfile. When you are done reading this page, refer to the Dockerfile Best Practices for a tip-oriented guide.

Usage

The docker build command builds an image from a Dockerfile and a context. The build’s context is the files at a specified location PATH or URL. The PATH is a directory on your local filesystem. The URL is a the location of a Git repository.

A context is processed recursively. So, a PATH includes any subdirectories and the URL includes the repository and its submodules. A simple build command that uses the current directory as context:

$ docker build .
Sending build context to Docker daemon  6.51 MB
...

The build is run by the Docker daemon, not by the CLI. The first thing a build process does is send the entire context (recursively) to the daemon. In most cases, it’s best to start with an empty directory as context and keep your Dockerfile in that directory. Add only the files needed for building the Dockerfile.

Warning: Do not use your root directory, /, as the PATH as it causes the build to transfer the entire contents of your hard drive to the Docker daemon.

To use a file in the build context, the Dockerfile refers to the file specified in an instruction, for example, a COPY instruction. To increase the build’s performance, exclude files and directories by adding a .dockerignore file to the context directory. For information about how to create a .dockerignore file see the documentation on this page.

Traditionally, the Dockerfile is called Dockerfile and located in the root of the context. You use the -f flag with docker build to point to a Dockerfile anywhere in your file system.

$ docker build -f /path/to/a/Dockerfile .

You can specify a repository and tag at which to save the new image if the build succeeds:

$ docker build -t shykes/myapp .

To tag the image into multiple repositories after the build, add multiple -t parameters when you run the build command:

$ docker build -t shykes/myapp:1.0.2 -t shykes/myapp:latest .

The Docker daemon runs the instructions in the Dockerfile one-by-one, committing the result of each instruction to a new image if necessary, before finally outputting the ID of your new image. The Docker daemon will automatically clean up the context you sent.

Note that each instruction is run independently, and causes a new image to be created - so RUN cd /tmp will not have any effect on the next instructions.

Whenever possible, Docker will re-use the intermediate images (cache), to accelerate the docker build process significantly. This is indicated by the Using cache message in the console output. (For more information, see the Build cache section) in the Dockerfile best practices guide:

$ docker build -t svendowideit/ambassador .
Sending build context to Docker daemon 15.36 kB
Step 0 : FROM alpine:3.2
 ---> 31f630c65071
Step 1 : MAINTAINER SvenDowideit@home.org.au
 ---> Using cache
 ---> 2a1c91448f5f
Step 2 : RUN apk update &&      apk add socat &&        rm -r /var/cache/
 ---> Using cache
 ---> 21ed6e7fbb73
Step 3 : CMD env | grep _TCP= | (sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat -t 100000000 TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/' && echo wait) | sh
 ---> Using cache
 ---> 7ea8aef582cc
Successfully built 7ea8aef582cc

When you’re done with your build, you’re ready to look into Pushing a repository to its registry.

Format

Here is the format of the Dockerfile:

# Comment
INSTRUCTION arguments

The instruction is not case-sensitive, however convention is for them to be UPPERCASE in order to distinguish them from arguments more easily.

Docker runs the instructions in a Dockerfile in order. The first instruction must be `FROM` in order to specify the Base Image from which you are building.

Docker will treat lines that begin with # as a comment. A # marker anywhere else in the line will be treated as an argument. This allows statements like:

# Comment
RUN echo 'we are running some # of cool things'

Here is the set of instructions you can use in a Dockerfile for building images.

Environment replacement

Environment variables (declared with the ENV statement) can also be used in certain instructions as variables to be interpreted by the Dockerfile. Escapes are also handled for including variable-like syntax into a statement literally.

Environment variables are notated in the Dockerfile either with $variable_name or ${variable_name}. They are treated equivalently and the brace syntax is typically used to address issues with variable names with no whitespace, like ${foo}_bar.

The ${variable_name} syntax also supports a few of the standard bash modifiers as specified below:

• ${variable:-word} indicates that if variable is set then the result will be that value. If variable is not set then word will be the result.
• ${variable:+word} indicates that if variable is set then word will be the result, otherwise the result is the empty string.

In all cases, word can be any string, including additional environment variables.

Escaping is possible by adding a \ before the variable: \$foo or \${foo}, for example, will translate to $foo and ${foo} literals respectively.

Example (parsed representation is displayed after the #):

FROM busybox
ENV foo /bar
WORKDIR ${foo}   # WORKDIR /bar
ADD . $foo       # ADD . /bar
COPY \$foo /quux # COPY $foo /quux

Environment variables are supported by the following list of instructions in the Dockerfile:

• ADD
• COPY
• ENV
• EXPOSE
• LABEL
• USER
• WORKDIR
• VOLUME
• STOPSIGNAL

as well as:

• ONBUILD (when combined with one of the supported instructions above)

Note: prior to 1.4, ONBUILD instructions did NOT support environment variable, even when combined with any of the instructions listed above.

Environment variable substitution will use the same value for each variable throughout the entire command. In other words, in this example:

ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc

will result in def having a value of hello, not bye. However, ghi will have a value of bye because it is not part of the same command that set abc to bye.

.dockerignore file

Before the docker CLI sends the context to the docker daemon, it looks for a file named .dockerignore in the root directory of the context. If this file exists, the CLI modifies the context to exclude files and directories that match patterns in it. This helps to avoid unnecessarily sending large or sensitive files and directories to the daemon and potentially adding them to images using ADD or COPY.

The CLI interprets the .dockerignore file as a newline-separated list of patterns similar to the file globs of Unix shells. For the purposes of matching, the root of the context is considered to be both the working and the root directory. For example, the patterns /foo/bar and foo/bar both exclude a file or directory named bar in the foo subdirectory of PATH or in the root of the git repository located at URL. Neither excludes anything else.

Here is an example .dockerignore file:

    */temp*
    */*/temp*
    temp?

This file causes the following build behavior:

Rule	Behavior
*/temp*	Exclude files and directories whose names start with temp in any immediate subdirectory of the root. For example, the plain file /somedir/temporary.txt is excluded, as is the directory /somedir/temp.
*/*/temp*	Exclude files and directories starting with temp from any subdirectory that is two levels below the root. For example, /somedir/subdir/temporary.txt is excluded.
temp?	Exclude files and directories in the root directory whose names are a one-character extension of temp. For example, /tempa and /tempb are excluded.

Matching is done using Go’s filepath.Match rules. A preprocessing step removes leading and trailing whitespace and eliminates . and .. elements using Go’s filepath.Clean. Lines that are blank after preprocessing are ignored.

Beyond Go’s filepath.Match rules, Docker also supports a special wildcard string ** that matches any number of directories (including zero). For example, **/*.go will exclude all files that end with .go that are found in all directories, including the root of the build context.

Lines starting with ! (exclamation mark) can be used to make exceptions to exclusions. The following is an example .dockerignore file that uses this mechanism:

    *.md
    !README.md

All markdown files except README.md are excluded from the context.

The placement of ! exception rules influences the behavior: the last line of the .dockerignore that matches a particular file determines whether it is included or excluded. Consider the following example:

    *.md
    !README*.md
    README-secret.md

No markdown files are included in the context except README files other than README-secret.md.

Now consider this example:

    *.md
    README-secret.md
    !README*.md

All of the README files are included. The middle line has no effect because !README*.md matches README-secret.md and comes last.

You can even use the .dockerignore file to exclude the Dockerfile and .dockerignore files. These files are still sent to the daemon because it needs them to do its job. But the ADD and COPY commands do not copy them to the image.

Finally, you may want to specify which files to include in the context, rather than which to exclude. To achieve this, specify * as the first pattern, followed by one or more ! exception patterns.

Note: For historical reasons, the pattern . is ignored.

FROM

FROM <image>

Or

FROM <image>:<tag>

Or

FROM <image>@<digest>

The FROM instruction sets the Base Image for subsequent instructions. As such, a valid Dockerfile must have FROM as its first instruction. The image can be any valid image – it is especially easy to start by pulling an image from the Public Repositories.

• FROM must be the first non-comment instruction in the Dockerfile.

• FROM can appear multiple times within a single Dockerfile in order to create multiple images. Simply make a note of the last image ID output by the commit before each new FROM command.

• The tag or digest values are optional. If you omit either of them, the builder assumes a latest by default. The builder returns an error if it cannot match the tag value.

MAINTAINER

MAINTAINER <name>

The MAINTAINER instruction allows you to set the Author field of the generated images.

RUN

RUN has 2 forms:

• RUN <command> (shell form, the command is run in a shell - /bin/sh -c)
• RUN ["executable", "param1", "param2"] (exec form)

The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.

Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image’s history, much like source control.

The exec form makes it possible to avoid shell string munging, and to RUN commands using a base image that does not contain /bin/sh.

In the shell form you can use a \ (backslash) to continue a single RUN instruction onto the next line. For example, consider these two lines:

RUN /bin/bash -c 'source $HOME/.bashrc ;\
echo $HOME'

Together they are equivalent to this single line:

RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'

Note: To use a different shell, other than ‘/bin/sh’, use the exec form passing in the desired shell. For example, RUN ["/bin/bash", "-c", "echo hello"]

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, RUN [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: RUN [ "sh", "-c", "echo $HOME" ].

Note: In the JSON form, it is necessary to escape backslashes. This is particularly relevant on Windows where the backslash is the path seperator. The following line would otherwise be treated as shell form due to not being valid JSON, and fail in an unexpected way: RUN ["c:\windows\system32\tasklist.exe"] The correct syntax for this example is: RUN ["c:\\windows\\system32\\tasklist.exe"]

The cache for RUN instructions isn’t invalidated automatically during the next build. The cache for an instruction like RUN apt-get dist-upgrade -y will be reused during the next build. The cache for RUN instructions can be invalidated by using the --no-cache flag, for example docker build --no-cache.

See the Dockerfile Best Practices guide for more information.

The cache for RUN instructions can be invalidated by ADD instructions. See below for details.

Known issues (RUN)

• Issue 783 is about file permissions problems that can occur when using the AUFS file system. You might notice it during an attempt to rm a file, for example.

For systems that have recent aufs version (i.e., dirperm1 mount option can be set), docker will attempt to fix the issue automatically by mounting the layers with dirperm1 option. More details on dirperm1 option can be found at aufs man page

If your system doesn’t have support for dirperm1, the issue describes a workaround.

CMD

The CMD instruction has three forms:

• CMD ["executable","param1","param2"] (exec form, this is the preferred form)
• CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
• CMD command param1 param2 (shell form)

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.

The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction as well.

Note: If CMD is used to provide default arguments for the ENTRYPOINT instruction, both the CMD and ENTRYPOINT instructions should be specified with the JSON array format.

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

When used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image.

If you use the shell form of the CMD, then the <command> will execute in /bin/sh -c:

FROM ubuntu
CMD echo "This is a test." | wc -

If you want to run your <command> without a shell then you must express the command as a JSON array and give the full path to the executable. This array form is the preferred format of CMD. Any additional parameters must be individually expressed as strings in the array:

FROM ubuntu
CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every time, then you should consider using ENTRYPOINT in combination with CMD. See ENTRYPOINT.

If the user specifies arguments to docker run then they will override the default specified in CMD.

Note: don’t confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.

LABEL

LABEL <key>=<value> <key>=<value> <key>=<value> ...

The LABEL instruction adds metadata to an image. A LABEL is a key-value pair. To include spaces within a LABEL value, use quotes and backslashes as you would in command-line parsing. A few usage examples:

LABEL "com.example.vendor"="ACME Incorporated"
LABEL com.example.label-with-value="foo"
LABEL version="1.0"
LABEL description="This text illustrates \
that label-values can span multiple lines."

An image can have more than one label. To specify multiple labels, Docker recommends combining labels into a single LABEL instruction where possible. Each LABEL instruction produces a new layer which can result in an inefficient image if you use many labels. This example results in a single image layer.

LABEL multi.label1="value1" multi.label2="value2" other="value3"

The above can also be written as:

LABEL multi.label1="value1" \
      multi.label2="value2" \
      other="value3"

Labels are additive including LABELs in FROM images. If Docker encounters a label/key that already exists, the new value overrides any previous labels with identical keys.

To view an image’s labels, use the docker inspect command.

"Labels": {
    "com.example.vendor": "ACME Incorporated"
    "com.example.label-with-value": "foo",
    "version": "1.0",
    "description": "This text illustrates that label-values can span multiple lines.",
    "multi.label1": "value1",
    "multi.label2": "value2",
    "other": "value3"
},

EXPOSE

EXPOSE <port> [<port>...]

The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. EXPOSE does not make the ports of the container accessible to the host. To do that, you must use either the -p flag to publish a range of ports or the -P flag to publish all of the exposed ports. You can expose one port number and publish it externally under another number.

To set up port redirection on the host system, see using the -P flag. The Docker network feature supports creating networks without the need to expose ports within the network, for detailed information see the overview of this feature).

ENV

ENV <key> <value>
ENV <key>=<value> ...

The ENV instruction sets the environment variable <key> to the value <value>. This value will be in the environment of all “descendant” Dockerfile commands and can be replaced inline in many as well.

The ENV instruction has two forms. The first form, ENV <key> <value>, will set a single variable to a value. The entire string after the first space will be treated as the <value> - including characters such as spaces and quotes.

The second form, ENV <key>=<value> ..., allows for multiple variables to be set at one time. Notice that the second form uses the equals sign (=) in the syntax, while the first form does not. Like command line parsing, quotes and backslashes can be used to include spaces within values.

For example:

ENV myName="John Doe" myDog=Rex\ The\ Dog \
    myCat=fluffy

and

ENV myName John Doe
ENV myDog Rex The Dog
ENV myCat fluffy

will yield the same net results in the final container, but the first form is preferred because it produces a single cache layer.

The environment variables set using ENV will persist when a container is run from the resulting image. You can view the values using docker inspect, and change them using docker run --env <key>=<value>.

Note: Environment persistence can cause unexpected side effects. For example, setting ENV DEBIAN_FRONTEND noninteractive may confuse apt-get users on a Debian-based image. To set a value for a single command, use RUN <key>=<value> <command>.

ADD

ADD has two forms:

• ADD <src>... <dest>
• ADD ["<src>",... "<dest>"] (this form is required for paths containing whitespace)

The ADD instruction copies new files, directories or remote file URLs from <src> and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resource may be specified but if they are files or directories then they must be relative to the source directory that is being built (the context of the build).

Each <src> may contain wildcards and matching will be done using Go’s filepath.Match rules. For example:

ADD hom* /mydir/        # adds all files starting with "hom"
ADD hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The <dest> is an absolute path, or a path relative to WORKDIR, into which the source will be copied inside the destination container.

ADD test relativeDir/          # adds "test" to `WORKDIR`/relativeDir/
ADD test /absoluteDir/         # adds "test" to /absoluteDir/

All new files and directories are created with a UID and GID of 0.

In the case where <src> is a remote file URL, the destination will have permissions of 600. If the remote file being retrieved has an HTTP Last-Modified header, the timestamp from that header will be used to set the mtime on the destination file. However, like any other file processed during an ADD, mtime will not be included in the determination of whether or not the file has changed and the cache should be updated.

Note: If you build by passing a Dockerfile through STDIN (docker build - < somefile), there is no build context, so the Dockerfile can only contain a URL based ADD instruction. You can also pass a compressed archive through STDIN: (docker build - < archive.tar.gz), the Dockerfile at the root of the archive and the rest of the archive will get used at the context of the build.

Note: If your URL files are protected using authentication, you will need to use RUN wget, RUN curl or use another tool from within the container as the ADD instruction does not support authentication.

Note: The first encountered ADD instruction will invalidate the cache for all following instructions from the Dockerfile if the contents of <src> have changed. This includes invalidating the cache for RUN instructions. See the Dockerfile Best Practices guide for more information.

ADD obeys the following rules:

• The <src> path must be inside the context of the build; you cannot ADD ../something /something, because the first step of a docker build is to send the context directory (and subdirectories) to the docker daemon.

• If <src> is a URL and <dest> does not end with a trailing slash, then a file is downloaded from the URL and copied to <dest>.

• If <src> is a URL and <dest> does end with a trailing slash, then the filename is inferred from the URL and the file is downloaded to <dest>/<filename>. For instance, ADD http://example.com/foobar / would create the file /foobar. The URL must have a nontrivial path so that an appropriate filename can be discovered in this case (http://example.com will not work).

• If <src> is a directory, the entire contents of the directory are copied, including filesystem metadata.

Note: The directory itself is not copied, just its contents.

• If <src> is a local tar archive in a recognized compression format (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources from remote URLs are not decompressed. When a directory is copied or unpacked, it has the same behavior as tar -x: the result is the union of:

  1. Whatever existed at the destination path and
  2. The contents of the source tree, with conflicts resolved in favor of “2.” on a file-by-file basis.

Note: Whether a file is identified as a recognized compression format or not is done solely based on the contents of the file, not the name of the file. For example, if an empty file happens to end with .tar.gz this will not be recognized as a compressed file and will not generate any kind of decompression error message, rather the file will simply be copied to the destination.

• If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

• If multiple <src> resources are specified, either directly or due to the use of a wildcard, then <dest> must be a directory, and it must end with a slash /.

• If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

• If <dest> doesn’t exist, it is created along with all missing directories in its path.

COPY

COPY has two forms:

• COPY <src>... <dest>
• COPY ["<src>",... "<dest>"] (this form is required for paths containing whitespace)

The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resource may be specified but they must be relative to the source directory that is being built (the context of the build).

Each <src> may contain wildcards and matching will be done using Go’s filepath.Match rules. For example:

COPY hom* /mydir/        # adds all files starting with "hom"
COPY hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The <dest> is an absolute path, or a path relative to WORKDIR, into which the source will be copied inside the destination container.

COPY test relativeDir/   # adds "test" to `WORKDIR`/relativeDir/
COPY test /absoluteDir/  # adds "test" to /absoluteDir/

All new files and directories are created with a UID and GID of 0.

Note: If you build using STDIN (docker build - < somefile), there is no build context, so COPY can’t be used.

COPY obeys the following rules:

• The <src> path must be inside the context of the build; you cannot COPY ../something /something, because the first step of a docker build is to send the context directory (and subdirectories) to the docker daemon.

• If <src> is a directory, the entire contents of the directory are copied, including filesystem metadata.

Note: The directory itself is not copied, just its contents.

• If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

• If multiple <src> resources are specified, either directly or due to the use of a wildcard, then <dest> must be a directory, and it must end with a slash /.

• If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

• If <dest> doesn’t exist, it is created along with all missing directories in its path.

ENTRYPOINT

ENTRYPOINT has two forms:

• ENTRYPOINT ["executable", "param1", "param2"] (exec form, preferred)
• ENTRYPOINT command param1 param2 (shell form)

An ENTRYPOINT allows you to configure a container that will run as an executable.

For example, the following will start nginx with its default content, listening on port 80:

docker run -i -t --rm -p 80:80 nginx

Command line arguments to docker run <image> will be appended after all elements in an exec form ENTRYPOINT, and will override all elements specified using CMD. This allows arguments to be passed to the entry point, i.e., docker run <image> -d will pass the -d argument to the entry point. You can override the ENTRYPOINT instruction using the docker run --entrypoint flag.

The shell form prevents any CMD or run command line arguments from being used, but has the disadvantage that your ENTRYPOINT will be started as a subcommand of /bin/sh -c, which does not pass signals. This means that the executable will not be the container’s PID 1 - and will not receive Unix signals - so your executable will not receive a SIGTERM from docker stop <container>.

Only the last ENTRYPOINT instruction in the Dockerfile will have an effect.

Exec form ENTRYPOINT example

You can use the exec form of ENTRYPOINT to set fairly stable default commands and arguments and then use either form of CMD to set additional defaults that are more likely to be changed.

FROM ubuntu
ENTRYPOINT ["top", "-b"]
CMD ["-c"]

When you run the container, you can see that top is the only process:

$ docker run -it --rm --name test  top -H
top - 08:25:00 up  7:27,  0 users,  load average: 0.00, 0.01, 0.05
Threads:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.1 us,  0.1 sy,  0.0 ni, 99.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   2056668 total,  1616832 used,   439836 free,    99352 buffers
KiB Swap:  1441840 total,        0 used,  1441840 free.  1324440 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
    1 root      20   0   19744   2336   2080 R  0.0  0.1   0:00.04 top

To examine the result further, you can use docker exec:

$ docker exec -it test ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  2.6  0.1  19752  2352 ?        Ss+  08:24   0:00 top -b -H
root         7  0.0  0.1  15572  2164 ?        R+   08:25   0:00 ps aux

And you can gracefully request top to shut down using docker stop test.

The following Dockerfile shows using the ENTRYPOINT to run Apache in the foreground (i.e., as PID 1):

FROM debian:stable
RUN apt-get update && apt-get install -y --force-yes apache2
EXPOSE 80 443
VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"]
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]

If you need to write a starter script for a single executable, you can ensure that the final executable receives the Unix signals by using exec and gosu commands:

#!/bin/bash
set -e

if [ "$1" = 'postgres' ]; then
    chown -R postgres "$PGDATA"

    if [ -z "$(ls -A "$PGDATA")" ]; then
        gosu postgres initdb
    fi

    exec gosu postgres "$@"
fi

exec "$@"

Lastly, if you need to do some extra cleanup (or communicate with other containers) on shutdown, or are co-ordinating more than one executable, you may need to ensure that the ENTRYPOINT script receives the Unix signals, passes them on, and then does some more work:

#!/bin/sh
# Note: I've written this using sh so it works in the busybox container too

# USE the trap if you need to also do manual cleanup after the service is stopped,
#     or need to start multiple services in the one container
trap "echo TRAPed signal" HUP INT QUIT TERM

# start service in background here
/usr/sbin/apachectl start

echo "[hit enter key to exit] or run 'docker stop <container>'"
read

# stop service and clean up here
echo "stopping apache"
/usr/sbin/apachectl stop

echo "exited $0"

If you run this image with docker run -it --rm -p 80:80 --name test apache, you can then examine the container’s processes with docker exec, or docker top, and then ask the script to stop Apache:

$ docker exec -it test ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.0   4448   692 ?        Ss+  00:42   0:00 /bin/sh /run.sh 123 cmd cmd2
root        19  0.0  0.2  71304  4440 ?        Ss   00:42   0:00 /usr/sbin/apache2 -k start
www-data    20  0.2  0.2 360468  6004 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
www-data    21  0.2  0.2 360468  6000 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
root        81  0.0  0.1  15572  2140 ?        R+   00:44   0:00 ps aux
$ docker top test
PID                 USER                COMMAND
10035               root                {run.sh} /bin/sh /run.sh 123 cmd cmd2
10054               root                /usr/sbin/apache2 -k start
10055               33                  /usr/sbin/apache2 -k start
10056               33                  /usr/sbin/apache2 -k start
$ /usr/bin/time docker stop test
test
real	0m 0.27s
user	0m 0.03s
sys	0m 0.03s

Note: you can over ride the ENTRYPOINT setting using --entrypoint, but this can only set the binary to exec (no sh -c will be used).

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, ENTRYPOINT [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: ENTRYPOINT [ "sh", "-c", "echo", "$HOME" ]. Variables that are defined in the Dockerfileusing ENV, will be substituted by the Dockerfile parser.

Shell form ENTRYPOINT example

You can specify a plain string for the ENTRYPOINT and it will execute in /bin/sh -c. This form will use shell processing to substitute shell environment variables, and will ignore any CMD or docker run command line arguments. To ensure that docker stop will signal any long running ENTRYPOINT executable correctly, you need to remember to start it with exec:

FROM ubuntu
ENTRYPOINT exec top -b

When you run this image, you’ll see the single PID 1 process:

$ docker run -it --rm --name test top
Mem: 1704520K used, 352148K free, 0K shrd, 0K buff, 140368121167873K cached
CPU:   5% usr   0% sys   0% nic  94% idle   0% io   0% irq   0% sirq
Load average: 0.08 0.03 0.05 2/98 6
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
    1     0 root     R     3164   0%   0% top -b

Which will exit cleanly on docker stop:

$ /usr/bin/time docker stop test
test
real    0m 0.20s
user    0m 0.02s
sys 0m 0.04s

If you forget to add exec to the beginning of your ENTRYPOINT:

FROM ubuntu
ENTRYPOINT top -b
CMD --ignored-param1

You can then run it (giving it a name for the next step):

$ docker run -it --name test top --ignored-param2
Mem: 1704184K used, 352484K free, 0K shrd, 0K buff, 140621524238337K cached
CPU:   9% usr   2% sys   0% nic  88% idle   0% io   0% irq   0% sirq
Load average: 0.01 0.02 0.05 2/101 7
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
    1     0 root     S     3168   0%   0% /bin/sh -c top -b cmd cmd2
    7     1 root     R     3164   0%   0% top -b

You can see from the output of top that the specified ENTRYPOINT is not PID 1.

If you then run docker stop test, the container will not exit cleanly - the stop command will be forced to send a SIGKILL after the timeout:

$ docker exec -it test ps aux
PID   USER     COMMAND
    1 root     /bin/sh -c top -b cmd cmd2
    7 root     top -b
    8 root     ps aux
$ /usr/bin/time docker stop test
test
real    0m 10.19s
user    0m 0.04s
sys 0m 0.03s

Understand how CMD and ENTRYPOINT interact

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container. There are few rules that describe their co-operation.

1. Dockerfile should specify at least one of CMD or ENTRYPOINT commands.

2. ENTRYPOINT should be defined when using the container as an executable.

3. CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.

4. CMD will be overridden when running the container with alternative arguments.

The table below shows what command is executed for different ENTRYPOINT / CMD combinations:

	No ENTRYPOINT	ENTRYPOINT exec_entry p1_entry	ENTRYPOINT [“exec_entry”, “p1_entry”]
No CMD	error, not allowed	/bin/sh -c exec_entry p1_entry	exec_entry p1_entry
CMD [“exec_cmd”, “p1_cmd”]	exec_cmd p1_cmd	/bin/sh -c exec_entry p1_entry exec_cmd p1_cmd	exec_entry p1_entry exec_cmd p1_cmd
CMD [“p1_cmd”, “p2_cmd”]	p1_cmd p2_cmd	/bin/sh -c exec_entry p1_entry p1_cmd p2_cmd	exec_entry p1_entry p1_cmd p2_cmd
CMD exec_cmd p1_cmd	/bin/sh -c exec_cmd p1_cmd	/bin/sh -c exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd	exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd

VOLUME

VOLUME ["/data"]

The VOLUME instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers. The value can be a JSON array, VOLUME ["/var/log/"], or a plain string with multiple arguments, such as VOLUME /var/log or VOLUME /var/log /var/db. For more information/examples and mounting instructions via the Docker client, refer to Share Directories via Volumes documentation.

The docker run command initializes the newly created volume with any data that exists at the specified location within the base image. For example, consider the following Dockerfile snippet:

FROM ubuntu
RUN mkdir /myvol
RUN echo "hello world" > /myvol/greeting
VOLUME /myvol

This Dockerfile results in an image that causes docker run, to create a new mount point at /myvol and copy the greeting file into the newly created volume.

Note: If any build steps change the data within the volume after it has been declared, those changes will be discarded.

Note: The list is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

USER

USER daemon

The USER instruction sets the user name or UID to use when running the image and for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.

WORKDIR

WORKDIR /path/to/workdir

The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile. If the WORKDIR doesn’t exist, it will be created even if its not used in any subsequent Dockerfile instruction.

It can be used multiple times in the one Dockerfile. If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction. For example:

WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd

The output of the final pwd command in this Dockerfile would be /a/b/c.

The WORKDIR instruction can resolve environment variables previously set using ENV. You can only use environment variables explicitly set in the Dockerfile. For example:

ENV DIRPATH /path
WORKDIR $DIRPATH/$DIRNAME
RUN pwd

The output of the final pwd command in this Dockerfile would be /path/$DIRNAME

ARG

ARG <name>[=<default value>]

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag. If a user specifies a build argument that was not defined in the Dockerfile, the build outputs an error.

One or more build-args were not consumed, failing build.

The Dockerfile author can define a single variable by specifying ARG once or many variables by specifying ARG more than once. For example, a valid Dockerfile:

FROM busybox
ARG user1
ARG buildno
...

A Dockerfile author may optionally specify a default value for an ARG instruction:

FROM busybox
ARG user1=someuser
ARG buildno=1
...

If an ARG value has a default and if there is no value passed at build-time, the builder uses the default.

An ARG variable definition comes into effect from the line on which it is defined in the Dockerfile not from the argument’s use on the command-line or elsewhere. For example, consider this Dockerfile:

1 FROM busybox
2 USER ${user:-some_user}
3 ARG user
4 USER $user
...

A user builds this file by calling:

$ docker build --build-arg user=what_user Dockerfile

The USER at line 2 evaluates to some_user as the user variable is defined on the subsequent line 3. The USER at line 4 evaluates to what_user as user is defined and the what_user value was passed on the command line. Prior to its definition by an ARG instruction, any use of a variable results in an empty string.

Note: It is not recommended to use build-time variables for passing secrets like github keys, user credentials etc.

You can use an ARG or an ENV instruction to specify variables that are available to the RUN instruction. Environment variables defined using the ENV instruction always override an ARG instruction of the same name. Consider this Dockerfile with an ENV and ARG instruction.

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER v1.0.0
4 RUN echo $CONT_IMG_VER

Then, assume this image is built with this command:

$ docker build --build-arg CONT_IMG_VER=v2.0.1 Dockerfile

In this case, the RUN instruction uses v1.0.0 instead of the ARG setting passed by the user:v2.0.1 This behavior is similar to a shell script where a locally scoped variable overrides the variables passed as arguments or inherited from environment, from its point of definition.

Using the example above but a different ENV specification you can create more useful interactions between ARG and ENV instructions:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER ${CONT_IMG_VER:-v1.0.0}
4 RUN echo $CONT_IMG_VER

Unlike an ARG instruction, ENV values are always persisted in the built image. Consider a docker build without the --build-arg flag:

$ docker build Dockerfile

Using this Dockerfile example, CONT_IMG_VER is still persisted in the image but its value would be v1.0.0 as it is the default set in line 3 by the ENV instruction.

The variable expansion technique in this example allows you to pass arguments from the command line and persist them in the final image by leveraging the ENV instruction. Variable expansion is only supported for a limited set of Dockerfile instructions.

Docker has a set of predefined ARG variables that you can use without a corresponding ARG instruction in the Dockerfile.

• HTTP_PROXY
• http_proxy
• HTTPS_PROXY
• https_proxy
• FTP_PROXY
• ftp_proxy
• NO_PROXY
• no_proxy

To use these, simply pass them on the command line using the --build-arg <varname>=<value> flag.

Impact on build caching

ARG variables are not persisted into the built image as ENV variables are. However, ARG variables do impact the build cache in similar ways. If a Dockerfile defines an ARG variable whose value is different from a previous build, then a “cache miss” occurs upon first use of the ARG variable. The declaration of the ARG variable does not count as a use.

For example, consider these two Dockerfile:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo $CONT_IMG_VER

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo hello

If you specify --build-arg CONT_IMG_VER=<value> on the command line, in both cases, the specification on line 2 does not cause a cache miss; line 3 does cause a cache miss.ARG CONT_IMG_VER causes the RUN line to be identified as the same as running CONT_IMG_VER=<value> echo hello, so if the <value> changes, we get a cache miss.

Consider another example under the same command line:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER $CONT_IMG_VER
4 RUN echo $CONT_IMG_VER

In this example, the cache miss occurs on line 3. The miss happens because the variable’s value in the ENV references the ARG variable and that variable is changed through the command line. In this example, the ENV command causes the image to include the value.

If an ENV instruction overrides an ARG instruction of the same name, like this Dockerfile:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER hello
4 RUN echo $CONT_IMG_VER

Line 3 does not cause a cache miss because the value of CONT_IMG_VER is a constant (hello). As a result, the environment variables and values used on the RUN (line 4) doesn’t change between builds.

ONBUILD

ONBUILD [INSTRUCTION]

The ONBUILD instruction adds to the image a trigger instruction to be executed at a later time, when the image is used as the base for another build. The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the FROM instruction in the downstream Dockerfile.

Any build instruction can be registered as a trigger.

This is useful if you are building an image which will be used as a base to build other images, for example an application build environment or a daemon which may be customized with user-specific configuration.

For example, if your image is a reusable Python application builder, it will require application source code to be added in a particular directory, and it might require a build script to be called after that. You can’t just call ADD and RUN now, because you don’t yet have access to the application source code, and it will be different for each application build. You could simply provide application developers with a boilerplate Dockerfile to copy-paste into their application, but that is inefficient, error-prone and difficult to update because it mixes with application-specific code.

The solution is to use ONBUILD to register advance instructions to run later, during the next build stage.

Here’s how it works:

1. When it encounters an ONBUILD instruction, the builder adds a trigger to the metadata of the image being built. The instruction does not otherwise affect the current build.
2. At the end of the build, a list of all triggers is stored in the image manifest, under the key OnBuild. They can be inspected with the docker inspect command.
3. Later the image may be used as a base for a new build, using the FROM instruction. As part of processing the FROM instruction, the downstream builder looks for ONBUILD triggers, and executes them in the same order they were registered. If any of the triggers fail, the FROM instruction is aborted which in turn causes the build to fail. If all triggers succeed, the FROM instruction completes and the build continues as usual.
4. Triggers are cleared from the final image after being executed. In other words they are not inherited by “grand-children” builds.

For example you might add something like this:

[...]
ONBUILD ADD . /app/src
ONBUILD RUN /usr/local/bin/python-build --dir /app/src
[...]

Warning: Chaining ONBUILD instructions using ONBUILD ONBUILD isn’t allowed.

Warning: The ONBUILD instruction may not trigger FROM or MAINTAINER instructions.

STOPSIGNAL

STOPSIGNAL signal

The STOPSIGNAL instruction sets the system call signal that will be sent to the container to exit. This signal can be a valid unsigned number that matches a position in the kernel’s syscall table, for instance 9, or a signal name in the format SIGNAME, for instance SIGKILL.

Dockerfile examples

Below you can see some examples of Dockerfile syntax. If you’re interested in something more realistic, take a look at the list of Dockerization examples.

# Nginx
#
# VERSION               0.0.1

FROM      ubuntu
MAINTAINER Victor Vieux <victor@docker.com>

LABEL Description="This image is used to start the foobar executable" Vendor="ACME Products" Version="1.0"
RUN apt-get update && apt-get install -y inotify-tools nginx apache2 openssh-server

# Firefox over VNC
#
# VERSION               0.3

FROM ubuntu

# Install vnc, xvfb in order to create a 'fake' display and firefox
RUN apt-get update && apt-get install -y x11vnc xvfb firefox
RUN mkdir ~/.vnc
# Setup a password
RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
# Autostart firefox (might not be the best way, but it does the trick)
RUN bash -c 'echo "firefox" >> /.bashrc'

EXPOSE 5900
CMD    ["x11vnc", "-forever", "-usepw", "-create"]

# Multiple images example
#
# VERSION               0.1

FROM ubuntu
RUN echo foo > bar
# Will output something like ===> 907ad6c2736f

FROM ubuntu
RUN echo moo > oink
# Will output something like ===> 695d7793cbe4

# You᾿ll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.