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page_title: Best Practices for Writing Dockerfiles page_description: Hints, tips and guidelines for writing clean, reliable Dockerfiles page_keywords: Examples, Usage, base image, docker, documentation, dockerfile, best practices, hub, official repo

Best Practices for Writing Dockerfiles

Overview

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 youre 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 youre 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. “Ephemeral” here means 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

For faster uploading and efficiency during docker build, you should make use of a .dockerignore file to exclude files or directories from the build context and final image. For example, unless.git is needed by your build process or scripts, you should add it to .dockerignore, which can save many megabytes worth of upload time.

Avoid installing unnecessary packages

You should avoid installing extra or unnecessary packages just because they might be “nice to have.” For example, you dont 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.

Heres an example from the buildpack-deps image:

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

The Dockerfile instructions

This section contains specific recommendations for the correct usage of the various instructions contained in a Dockerfile.

FROM

Whenever possible, use Official Repositories as the basis for your image. We recommend the Debian image since its very tightly controlled and kept extremely minimal (currently under 100 mb), while still being a full distribution.

RUN

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

Probably the most common use-case for RUN is an application of apt-get. When using apt-get, here a few things to keep in mind:

  • Dont do RUN apt-get update on a single line. This will cause caching issues if the referenced archive gets updated, which will make your subsequent apt-get install fail without comment.

  • For the most part, to keep your code more readable and maintainable, avoid RUN apt-get install -y package-foo && apt-get install -y package-bar.

  • Avoid RUN apt-get upgrade or dist-upgrade, since many of the “essential” packages from the base images will fail to upgrade inside an unprivileged container. If a base package is out of date, you should contact its maintainers. If you know theres a particular package, foo, that needs to be updated, use apt-get install -y foo and it will update automatically.

  • Do use RUN apt-get update && apt-get install -y package-bar package-foo package-baz. Writing the instruction this way not only makes it easier to read and maintain, but also, by including apt-get update, ensures that the cache will naturally be busted and the latest versions will be installed with no further coding or manual intervention required.

  • Further natural cache-busting can be realized by version-pinning packages (e.g., package-foo=1.3.*). This will force retrieval of that version regardless of whats in the cache. Forming your apt-get code this way will greatly ease maintenance and reduce failures due to unanticipated changes in required packages.

Example

Below is a well-formed RUN instruction that demonstrates the above recommendations. Note that the last package, s3cmd, specifies a version 1.1.0*. If the image previously used an older version, specifying the new one will cause a cache bust of apt-get update and ensure the installation of the new version (which in this case had a new, required feature).

RUN apt-get update && apt-get install -y \
    aufs-tools \
    automake \
    btrfs-tools \
    build-essential \
    curl \
    dpkg-sig \
    git \
    iptables \
    libapparmor-dev \
    libcap-dev \
    libsqlite3-dev \
    lxc=1.0* \
    mercurial \
    parallel \
    reprepro \
    ruby1.9.1 \
    ruby1.9.1-dev \
    s3cmd=1.1.0*

CMD

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, youll 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

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

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 Postgress 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

Although ADD and COPY are functionally similar, generally speaking, COPY is preferred. Thats because its 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 /.

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 theyve 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 mdkir -p /usr/src/things \
    && curl -SL http://example.com/big.tar.gz \
    | tar -xJC /usr/src/things \
    && make -C /usr/src/things all

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

ENTRYPOINT

The best use for ENTRYPOINT is as a helper script. Using ENTRYPOINT for other tasks can make your code harder to understand. For example, docker run -it official-image bash is much easier to understand than docker run -it --entrypoint bash official-image -i, especially for Docker beginners.

In order to avoid a situation where commands are run without clear visibility to the user, make sure your script ends with something like exec "$@". After the entrypoint completes, the script will transparently bootstrap the command invoked by the user, making what has been run clear to the user (for example, docker run -it mysql mysqld --some --flags will transparently run mysqld --some --flags after ENTRYPOINT runs initdb).

For example, lets look at the Dockerfile for the Postgres Official Image. It refers to the following script:

#!/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 "$@"

That script then gets copied into the container and the run via ENTRYPOINT on container startup:

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

VOLUME

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

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

that users/groups in an image get assigned a non-deterministic UID/GID in that the “next” UID/GID gets assigned regardless of image rebuilds. So, if its 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 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, try to minimize switching USER back and forth frequently.

WORKDIR

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

ONBUILD is only 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 Rubys 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 Repos have exemplary Dockerfiles:

Additional Resources: