Best practices for Dockerfile instructions | Docker Docs

Excerpt

Hints, tips and guidelines for writing clean, reliable Dockerfile instructions

These recommendations are designed to help you create an efficient and maintainable Dockerfile.

FROM

Whenever possible, use current official images as the basis for your images. Docker recommends the Alpine imageopen_in_new as it is tightly controlled and small in size (currently under 6 MB), while still being a full Linux distribution.

For more information about the FROM instruction, see Dockerfile reference for the FROM instruction.

LABEL

You can add labels to your image to help organize images by project, record licensing information, to aid in automation, or for other reasons. For each label, add a line beginning with LABEL with one or more key-value pairs. The following examples show the different acceptable formats. Explanatory comments are included inline.

Strings with spaces must be quoted or the spaces must be escaped. Inner quote characters ("), must also be escaped. For example:

# Set one or more individual labels
LABEL com.example.version="0.0.1-beta"
LABEL vendor1="ACME Incorporated"
LABEL vendor2=ZENITH\ Incorporated
LABEL com.example.release-date="2015-02-12"
LABEL com.example.version.is-production=""

An image can have more than one label. Prior to Docker 1.10, it was recommended to combine all labels into a single LABEL instruction, to prevent extra layers from being created. This is no longer necessary, but combining labels is still supported. For example:

# Set multiple labels on one line
LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"

The above example can also be written as:

# Set multiple labels at once, using line-continuation characters to break long lines
LABEL vendor=ACME\ Incorporated \
      com.example.is-beta= \
      com.example.is-production="" \
      com.example.version="0.0.1-beta" \
      com.example.release-date="2015-02-12"

See Understanding object labels for guidelines about acceptable label keys and values. For information about querying labels, refer to the items related to filtering in Managing labels on objects. See also LABEL in the Dockerfile reference.

RUN

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

For more information about RUN, see Dockerfile reference for the RUN instruction.

apt-get

Probably the most common use case for RUN is an application of apt-get. Because it installs packages, the RUN apt-get command has several counter-intuitive behaviors to look out for.

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  \
    && rm -rf /var/lib/apt/lists/*

Using apt-get update alone in a RUN statement causes caching issues and subsequent apt-get install instructions to fail. For example, this issue will occur in the following Dockerfile:

# syntax=docker/dockerfile:1

FROM ubuntu:22.04
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 an extra package as shown in the following Dockerfile:

# syntax=docker/dockerfile:1

FROM ubuntu:22.04
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 isn’t executed because the build uses the cached version. Because the apt-get update isn’t 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 argument specifies a version 1.1.*. If the image previously used an older version, specifying the new one causes a cache bust of apt-get update and ensures the installation of the new version. Listing packages on each line can also prevent mistakes in package duplication.

In addition, when you clean up the apt cache by removing /var/lib/apt/lists it reduces the image size, since the apt cache isn’t stored in a layer. Since the RUN statement starts with apt-get update, the package cache is always refreshed prior to apt-get install.

Official Debian and Ubuntu images automatically run apt-get cleanopen_in_new, so explicit invocation is not required.

Using pipes

Some RUN commands depend on the ability to pipe the output of one command into another, using the pipe character (|), as in the following example:

RUN wget -O - https://some.site | wc -l > /number

Docker executes these commands using the /bin/sh -c interpreter, which only evaluates the exit code of the last operation in the pipe to determine success. In the example above, this build step succeeds and produces a new image so long as the wc -l command succeeds, even if the wget command fails.

If you want the command to fail due to an error at any stage in the pipe, prepend set -o pipefail && to ensure that an unexpected error prevents the build from inadvertently succeeding. For example:

RUN set -o pipefail && wget -O - https://some.site | wc -l > /number

Note

Not all shells support the -o pipefail option.

In cases such as the dash shell on Debian-based images, consider using the exec form of RUN to explicitly choose a shell that does support the pipefail option. For example:
RUN ["/bin/bash", "-c", "set -o pipefail && wget -O - https://some.site | wc -l > /number"]

CMD

The CMD instruction should be used to run the software contained in 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, such as Apache and Rails, 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, such as bash, python and perl. 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.

For more information about CMD, see Dockerfile reference for the CMD instruction.

EXPOSE

The EXPOSE instruction indicates the ports on which a container listens 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).

For more information about EXPOSE, see Dockerfile reference for the EXPOSE instruction.

ENV

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 ensures that CMD ["nginx"] just works.

The ENV instruction is also useful for providing the 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 https://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgres && …
ENV PATH=/usr/local/postgres-$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 automatically bump the version of the software in your container.

Each ENV line creates a new intermediate layer, just like RUN commands. This means that even if you unset the environment variable in a future layer, it still persists in this layer and its value can be dumped. You can test this by creating a Dockerfile like the following, and then building it.

# syntax=docker/dockerfile:1
FROM alpine
ENV ADMIN_USER="mark"
RUN echo $ADMIN_USER > ./mark
RUN unset ADMIN_USER

$ docker run --rm test sh -c 'echo $ADMIN_USER'

mark

To prevent this, and really unset the environment variable, use a RUN command with shell commands, to set, use, and unset the variable all in a single layer. You can separate your commands with ; or &&. If you use the second method, and one of the commands fails, the docker build also fails. This is usually a good idea. Using \ as a line continuation character for Linux Dockerfiles improves readability. You could also put all of the commands into a shell script and have the RUN command just run that shell script.

# syntax=docker/dockerfile:1
FROM alpine
RUN export ADMIN_USER="mark" \
    && echo $ADMIN_USER > ./mark \
    && unset ADMIN_USER
CMD sh

$ docker run --rm test sh -c 'echo $ADMIN_USER'

For more information about ENV, see Dockerfile reference for the ENV instruction.

ADD or COPY

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. If a specifically required file changes, then this ensures that only that step’s build cache is invalidated, forcing only that step to be run again.

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 don’t have to add another layer in your image. For example, you should avoid doing things like:

ADD https://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 https://example.com/big.tar.xz \
    | tar -xJC /usr/src/things \
    && make -C /usr/src/things all

For other items, like files and directories, that don’t require the tar auto-extraction capability of ADD, you should always use COPY.

For more information about ADD or COPY, see the following:

ENTRYPOINT

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.

The following is an example of an image for the command line tool s3cmd:

You can use the following command to run the image and show the command’s help:

Or, you can use the right parameters to execute a command, like in the following example:

$ 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 Imageopen_in_new 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 "$@"

This script uses the exec Bash commandopen_in_new 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. For more information, see the ENTRYPOINT reference.

In the following example, a helper script is copied into the container and run via ENTRYPOINT on container start:

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

It can simply start 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

For more information about ENTRYPOINT, see Dockerfile reference for the ENTRYPOINT instruction.

VOLUME

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

For more information about VOLUME, see Dockerfile reference for the VOLUME instruction.

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 the following example:

RUN groupadd -r postgres && useradd --no-log-init -r -g postgres postgres

Note

Consider an explicit UID/GID.

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

Note

Due to an unresolved bugopen_in_new in the Go archive/tar package’s handling of sparse files, attempting to create a user with a significantly large UID inside a Docker container can lead to disk exhaustion because /var/log/faillog in the container layer is filled with NULL (\0) characters. A workaround is to pass the --no-log-init flag to useradd. The Debian/Ubuntu adduser wrapper does not support this flag.

Avoid installing or using sudo as it has unpredictable TTY and signal-forwarding behavior that can cause problems. If you absolutely need functionality similar to sudo, such as initializing the daemon as root but running it as non-root, consider using “gosu”open_in_new.

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

For more information about USER, see Dockerfile reference for the USER instruction.

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.

For more information about WORKDIR, see Dockerfile reference for the WORKDIR instruction.

ONBUILD

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 that 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 variantsopen_in_new.

Images built with 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 fails catastrophically if the new build’s context is missing the resource being added. Adding a separate tag, as recommended above, helps mitigate this by allowing the Dockerfile author to make a choice.

For more information about ONBUILD, see Dockerfile reference for the ONBUILD instruction.

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