Best practices for writing Dockerfiles

Best practices for writing Dockerfiles

Docker can build images automatically by reading the instructions from a Dockerfile, a text file that contains all the commands, in order, needed to build a given image. Dockerfiles adhere to a specific format and use a specific set of instructions. You can learn the basics on the Dockerfile Reference page. If you’re new to writing Dockerfiles, you should start there.

This document covers the best practices and methods recommended by Docker, Inc. and the Docker community for creating easy-to-use, effective Dockerfiles. We strongly suggest you follow these recommendations (in fact, if you’re creating an Official Image, you must adhere to these practices).

You can see many of these practices and recommendations in action in the buildpack-deps Dockerfile.

Note: for more detailed explanations of any of the Dockerfile commands mentioned here, visit the Dockerfile Reference page.

General guidelines and recommendations

Containers should be ephemeral

The container produced by the image your Dockerfile defines should be as ephemeral as possible. By “ephemeral,” we mean that it can be stopped and destroyed and a new one built and put in place with an absolute minimum of set-up and configuration.

Use a .dockerignore file

In most cases, it’s best to put each Dockerfile in an empty directory. Then, add to that directory only the files needed for building the Dockerfile. To increase the build’s performance, you can exclude files and directories by adding a .dockerignore file to that directory as well. This file supports exclusion patterns similar to .gitignore files. For information on creating one, see the .dockerignore file.

Avoid installing unnecessary packages

In order to reduce complexity, dependencies, file sizes, and build times, you should avoid installing extra or unnecessary packages just because they might be “nice to have.” For example, you don’t need to include a text editor in a database image.

Run only one process per container

In almost all cases, you should only run a single process in a single container. Decoupling applications into multiple containers makes it much easier to scale horizontally and reuse containers. If that service depends on another service, make use of container linking.

Minimize the number of layers

You need to find the balance between readability (and thus long-term maintainability) of the Dockerfile and minimizing the number of layers it uses. Be strategic and cautious about the number of layers you use.

Sort multi-line arguments

Whenever possible, ease later changes by sorting multi-line arguments alphanumerically. This will help you avoid duplication of packages and make the list much easier to update. This also makes PRs a lot easier to read and review. Adding a space before a backslash (\) helps as well.

Here’s an example from the buildpack-deps image:

RUN apt-get update && apt-get install -y \
  bzr \
  cvs \
  git \
  mercurial \
  subversion

Build cache

During the process of building an image Docker will step through the instructions in your Dockerfile executing each in the order specified. As each instruction is examined Docker will look for an existing image in its cache that it can reuse, rather than creating a new (duplicate) image. If you do not want to use the cache at all you can use the --no-cache=true option on the docker build command.

However, if you do let Docker use its cache then it is very important to understand when it will, and will not, find a matching image. The basic rules that Docker will follow are outlined below:

• Starting with a base image that is already in the cache, the next instruction is compared against all child images derived from that base image to see if one of them was built using the exact same instruction. If not, the cache is invalidated.

• In most cases simply comparing the instruction in the Dockerfile with one of the child images is sufficient. However, certain instructions require a little more examination and explanation.

• For the ADD and COPY instructions, the contents of the file(s) in the image are examined and a checksum is calculated for each file. The last-modified and last-accessed times of the file(s) are not considered in these checksums. During the cache lookup, the checksum is compared against the checksum in the existing images. If anything has changed in the file(s), such as the contents and metadata, then the cache is invalidated.

• Aside from the ADD and COPY commands, cache checking will not look at the files in the container to determine a cache match. For example, when processing a RUN apt-get -y update command the files updated in the container will not be examined to determine if a cache hit exists. In that case just the command string itself will be used to find a match.

Once the cache is invalidated, all subsequent Dockerfile commands will generate new images and the cache will not be used.

The Dockerfile instructions

Below you’ll find recommendations for the best way to write the various instructions available for use in a Dockerfile.

FROM

Dockerfile reference for the FROM instruction

Whenever possible, use current Official Repositories as the basis for your image. We recommend the Debian image since it’s very tightly controlled and kept minimal (currently under 150 mb), while still being a full distribution.

RUN

Dockerfile reference for the RUN instruction

As always, to make your Dockerfile more readable, understandable, and maintainable, split long or complex RUN statements on multiple lines separated with backslashes.

apt-get

Probably the most common use-case for RUN is an application of apt-get. The RUN apt-get command, because it installs packages, has several gotchas to look out for.

You should avoid RUN apt-get upgrade or dist-upgrade, as many of the “essential” packages from the base images won’t upgrade inside an unprivileged container. If a package contained in the base image is out-of-date, you should contact its maintainers. If you know there’s a particular package, foo, that needs to be updated, use apt-get install -y foo to update automatically.

Always combine RUN apt-get update with apt-get install in the same RUN statement, for example:

    RUN apt-get update && apt-get install -y \
        package-bar \
        package-baz \
        package-foo

Using apt-get update alone in a RUN statement causes caching issues and subsequent apt-get install instructions fail. For example, say you have a Dockerfile:

    FROM ubuntu:14.04
    RUN apt-get update
    RUN apt-get install -y curl

After building the image, all layers are in the Docker cache. Suppose you later modify apt-get install by adding extra package:

    FROM ubuntu:14.04
    RUN apt-get update
    RUN apt-get install -y curl nginx

Docker sees the initial and modified instructions as identical and reuses the cache from previous steps. As a result the apt-get update is NOT executed because the build uses the cached version. Because the apt-get update is not run, your build can potentially get an outdated version of the curl and nginx packages.

Using RUN apt-get update && apt-get install -y ensures your Dockerfile installs the latest package versions with no further coding or manual intervention. This technique is known as “cache busting”. You can also achieve cache-busting by specifying a package version. This is known as version pinning, for example:

    RUN apt-get update && apt-get install -y \
        package-bar \
        package-baz \
        package-foo=1.3.*

Version pinning forces the build to retrieve a particular version regardless of what’s in the cache. This technique can also reduce failures due to unanticipated changes in required packages.

Below is a well-formed RUN instruction that demonstrates all the apt-get recommendations.

RUN apt-get update && apt-get install -y \
    aufs-tools \
    automake \
    build-essential \
    curl \
    dpkg-sig \
    libcap-dev \
    libsqlite3-dev \
    mercurial \
    reprepro \
    ruby1.9.1 \
    ruby1.9.1-dev \
    s3cmd=1.1.* \
 && rm -rf /var/lib/apt/lists/*

The s3cmd instructions specifies a version 1.1.0*. If the image previously used an older version, specifying the new one causes a cache bust of apt-get update and ensure the installation of the new version. Listing packages on each line can also prevent mistakes in package duplication.

In addition, cleaning up the apt cache and removing /var/lib/apt/lists helps keep the image size down. Since the RUN statement starts with apt-get update, the package cache will always be refreshed prior to apt-get install.

Note: The official Debian and Ubuntu images automatically run apt-get clean, so explicit invocation is not required.

CMD

Dockerfile reference for the CMD instruction

The CMD instruction should be used to run the software contained by your image, along with any arguments. CMD should almost always be used in the form of CMD [“executable”, “param1”, “param2”…]. Thus, if the image is for a service (Apache, Rails, etc.), you would run something like CMD ["apache2","-DFOREGROUND"]. Indeed, this form of the instruction is recommended for any service-based image.

In most other cases, CMD should be given an interactive shell (bash, python, perl, etc), for example, CMD ["perl", "-de0"], CMD ["python"], or CMD [“php”, “-a”]. Using this form means that when you execute something like docker run -it python, you’ll get dropped into a usable shell, ready to go. CMD should rarely be used in the manner of CMD [“param”, “param”] in conjunction with ENTRYPOINT, unless you and your expected users are already quite familiar with how ENTRYPOINT works.

EXPOSE

Dockerfile reference for the EXPOSE instruction

The EXPOSE instruction indicates the ports on which a container will listen for connections. Consequently, you should use the common, traditional port for your application. For example, an image containing the Apache web server would use EXPOSE 80, while an image containing MongoDB would use EXPOSE 27017 and so on.

For external access, your users can execute docker run with a flag indicating how to map the specified port to the port of their choice. For container linking, Docker provides environment variables for the path from the recipient container back to the source (ie, MYSQL_PORT_3306_TCP).

ENV

Dockerfile reference for the ENV instruction

In order to make new software easier to run, you can use ENV to update the PATH environment variable for the software your container installs. For example, ENV PATH /usr/local/nginx/bin:$PATH will ensure that CMD [“nginx”] just works.

The ENV instruction is also useful for providing required environment variables specific to services you wish to containerize, such as Postgres’s PGDATA.

Lastly, ENV can also be used to set commonly used version numbers so that version bumps are easier to maintain, as seen in the following example:

ENV PG_MAJOR 9.3
ENV PG_VERSION 9.3.4
RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH

Similar to having constant variables in a program (as opposed to hard-coding values), this approach lets you change a single ENV instruction to auto-magically bump the version of the software in your container.

ADD or COPY

Dockerfile reference for the ADD instruction
Dockerfile reference for the COPY instruction

Although ADD and COPY are functionally similar, generally speaking, COPY is preferred. That’s because it’s more transparent than ADD. COPY only supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support) that are not immediately obvious. Consequently, the best use for ADD is local tar file auto-extraction into the image, as in ADD rootfs.tar.xz /.

If you have multiple Dockerfile steps that use different files from your context, COPY them individually, rather than all at once. This will ensure that each step’s build cache is only invalidated (forcing the step to be re-run) if the specifically required files change.

For example:

COPY requirements.txt /tmp/
RUN pip install --requirement /tmp/requirements.txt
COPY . /tmp/

Results in fewer cache invalidations for the RUN step, than if you put the COPY . /tmp/ before it.

Because image size matters, using ADD to fetch packages from remote URLs is strongly discouraged; you should use curl or wget instead. That way you can delete the files you no longer need after they’ve been extracted and you won’t have to add another layer in your image. For example, you should avoid doing things like:

ADD http://example.com/big.tar.xz /usr/src/things/
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all

And instead, do something like:

RUN mkdir -p /usr/src/things \
    && curl -SL http://example.com/big.tar.xz \
    | tar -xJC /usr/src/things \
    && make -C /usr/src/things all

For other items (files, directories) that do not require ADD’s tar auto-extraction capability, you should always use COPY.

ENTRYPOINT

Dockerfile reference for the ENTRYPOINT instruction

The best use for ENTRYPOINT is to set the image’s main command, allowing that image to be run as though it was that command (and then use CMD as the default flags).

Let’s start with an example of an image for the command line tool s3cmd:

ENTRYPOINT ["s3cmd"]
CMD ["--help"]

Now the image can be run like this to show the command’s help:

$ docker run s3cmd

Or using the right parameters to execute a command:

$ docker run s3cmd ls s3://mybucket

This is useful because the image name can double as a reference to the binary as shown in the command above.

The ENTRYPOINT instruction can also be used in combination with a helper script, allowing it to function in a similar way to the command above, even when starting the tool may require more than one step.

For example, the Postgres Official Image uses the following script as its ENTRYPOINT:

#!/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 "$@"

Note: This script uses the exec Bash command so that the final running application becomes the container’s PID 1. This allows the application to receive any Unix signals sent to the container. See the ENTRYPOINT help for more details.

The helper script is copied into the container and run via ENTRYPOINT on container start:

COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]

This script allows the user to interact with Postgres in several ways.

It can simply start Postgres:

$ docker run postgres

Or, it can be used to run Postgres and pass parameters to the server:

$ docker run postgres postgres --help

Lastly, it could also be used to start a totally different tool, such as Bash:

$ docker run --rm -it postgres bash

VOLUME

Dockerfile reference for the VOLUME instruction

The VOLUME instruction should be used to expose any database storage area, configuration storage, or files/folders created by your docker container. You are strongly encouraged to use VOLUME for any mutable and/or user-serviceable parts of your image.

USER

Dockerfile reference for the USER instruction

If a service can run without privileges, use USER to change to a non-root user. Start by creating the user and group in the Dockerfile with something like RUN groupadd -r postgres && useradd -r -g postgres postgres.

Note: Users and groups in an image get a non-deterministic UID/GID in that the “next” UID/GID gets assigned regardless of image rebuilds. So, if it’s critical, you should assign an explicit UID/GID.

You should avoid installing or using sudo since it has unpredictable TTY and signal-forwarding behavior that can cause more problems than it solves. If you absolutely need functionality similar to sudo (e.g., initializing the daemon as root but running it as non-root), you may be able to use “gosu”.

Lastly, to reduce layers and complexity, avoid switching USER back and forth frequently.

WORKDIR

Dockerfile reference for the WORKDIR instruction

For clarity and reliability, you should always use absolute paths for your WORKDIR. Also, you should use WORKDIR instead of proliferating instructions like RUN cd … && do-something, which are hard to read, troubleshoot, and maintain.

ONBUILD

Dockerfile reference for the ONBUILD instruction

An ONBUILD command executes after the current Dockerfile build completes. ONBUILD executes in any child image derived FROM the current image. Think of the ONBUILD command as an instruction the parent Dockerfile gives to the child Dockerfile.

A Docker build executes ONBUILD commands before any command in a child Dockerfile.

ONBUILD is useful for images that are going to be built FROM a given image. For example, you would use ONBUILD for a language stack image that builds arbitrary user software written in that language within the Dockerfile, as you can see in Ruby’s ONBUILD variants.

Images built from ONBUILD should get a separate tag, for example: ruby:1.9-onbuild or ruby:2.0-onbuild.

Be careful when putting ADD or COPY in ONBUILD. The “onbuild” image will fail catastrophically if the new build’s context is missing the resource being added. Adding a separate tag, as recommended above, will help mitigate this by allowing the Dockerfile author to make a choice.

Examples for Official Repositories

These Official Repositories have exemplary Dockerfiles:

• Go
• Perl
• Hy
• Rails

Additional resources:

• Dockerfile Reference
• More about Base Images
• More about Automated Builds
• Guidelines for Creating Official Repositories