1. Understand build context

When executing "docker build", the current working directory is the build context. By default, the Dockerfile is assumed to be in the current working directory.

Irrespective of where the Dockerfile is located, all recursive contents of files and directories in the current directory are sent to the Docker daemon as the build context.

Inadvertently including files that are not necessary for building an image results in a larger build context and larger image size.

This can increase the time to build the image, time to pull and push it, and the container runtime size.

"this can increase'..."

I suggest the very first guideline should be a general statement about image size, as many of these individual items address that general concern

2. Exclude with .dockerignore

Exclude files not relevant to the build with a .dockerignore file.

This file supports exclusion patterns similar to .gitignore files.

3.Use multi-stage builds

Multi-stage builds reduces the size of an image, without worrying about the number of intermediate layers and files.

An image is built during the final stage of the build process. The number of image layers can be minimized by leveraging build cache.

For a build that contains several layers, order them from the less frequently changed (re-use build cache) to the more frequently changed:

• Install tools you need to build your application

• Install or update library dependencies

• Generate your application

4. Don’t install unnecessary packages

Avoid installing extra or unnecessary packages.

This will reduce complexity, dependencies, file sizes, and build times,  Don't include a text editor in a database image.

5. Decouple applications

Apply the principle of "separation of concerns."

Each container should have only one concern. Decoupling applications into multiple containers makes it easier to reuse containers.

6. Minimize the number of layers

The instructions RUN, COPY, ADD create layers and directly increase the size of the build.

Use multi-stage builds, to only copy the artifacts you need into the final image.

Tools and debug information can be added to intermediate build stages without increasing the size of the final image.

7. Sort multi-line arguments

To minimize duplication of packages and make the list of packages much easier to update, sort multi-line arguments alphanumerically.

8. Re-use the build cache

As each instruction in the Dockerfile is examined, the builder looks for an existing image in its cache that can be reused, rather than creating a duplicate image.

• 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 does 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 are not examined to determine if a cache hit exists. In that case just the command string itself is used to find a match.

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

FROM

Use current official repositories for base images. ONAP images must be vendor agnostic; ensure that the base images are cpu architecture-agnostic.

LABEL

Labels are unique key-value pairs used to add metadata to container images and containers. They help organize images by project, add licensing information, or to support build and CI pipeline.

An image can have more that one label. For each label, begin a new line with "LABEL" and add one or more key-value pairs.

RUN

To make the build file (e.g. Dockerfile) more readable, understandable and maintainable:

RUN apt-get

1. Split a long or complex statement into multiple lines

2. Separate each line by a backslash (\)

3. Avoid RUN apt-get upgrade and RUN apt-get dist-upgrade.

   1. Some packages from the parent image may not upgrade in a container.

4. If you must update a package (e.g. bar) use "apt-get install -y bar"

5. Install the latest package versions with no further coding or manual intervention by combining "RUN apt-get update" and "apt-get install -y" in a single RUN statement:

   1. RUN apt-get update && apt-get install -y

6. Use "version pinning." It will:

   1. Force the build to use a particular package version regardless of what's in the cache

   2. Reduce failures due to unexpected changes in required packages

Well-formatted example:

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/*

ERRORS IN STAGES OF A PIPE

7. Use "set -o pipefail &&"  to ensure that the RUN command only succeeds if all stages of a pipe succeed.

RUN ["/bin/bash", "-c", "set -o pipefail && wget -O - https://myapp.net | wc -l >= b"]

CMD

This instruction provides defaults to run the application packaged in a container image. It should always be used in the form:

CMD [“executable”, “arg1”, “arg2”…]

Typically, CMD should run an interactive shell. That way users get a usable shell when they execute "docker run -it ..." For example:

CMD ["python"]

CMD [“php”, “-a”]

Note: If the user provides arguments to "docker run", they override the defaults specified in "CMD."

EXPOSE

Use well-known ports for your application. For example, an image containing Apache web server should use EXPOSE 80. An image containing MongoDB should use EXPOSE 27017 and so on and so forth.

ENV

Use ENV to avoid hard-coding values for variables and parameters in the your build file. ENV can parameterize container variables. For example, the version of the software in the container (VERSION), the PATH environment variable and other execution environment variables (MAJOR).

ENV MAJOR 1.3
ENV VERSION 1.3.4
RUN curl -SL http://example.com/postgres-$VERSION.tar.xz | tar -xJC /usr/src/postgress && …
ENV PATH /usr/local/postgres-$MAJOR/bin:$PATH

Each ENV command creates a new intermediate layer. Even if you unset the environment variable in a future layer, it still persists in this layer. Use a RUN command with shell commands, to set, use, and unset the variable all in a single layer. Separate your commands with &&.

Example

RUN export ADMIN_USER="seneca" \
    && echo $ADMIN_USER > ./seneca \
    && unset ADMIN_USER

ADD or COPY

ADD is recommended for local tar file auto-extraction into the image (e.g. ADD rootfs.tar.xz /.).

If you have to copy several files from your context, COPY them individually, instead of all at once. That way each step is only re-execute if the specifically required files change.

To reduce the number of layers and the image size, don't use ADD to download packages from URLs. Use curl or wget and delete the files you no longer need after they’ve been extracted.

Example:

RUN mkdir -p /usr/src/ether \
    && curl -SL http://vacuum.com/huge.tar.xz \
    | tar -xJC /usr/src/ether \
    && make -C /usr/src/ether all

ENTRYPOINT

Use ENTRYPOINT to set the image’s main command. That allows the image to be executed as though it was that command. Use CMD to set the default arguments.

Example:

By setting

ENTRYPOINT ["my_command"]

CMD["–version"]

The image can be run as

$ docker run my_command

or

$ docker rum my_command --verbose -n 10

ENTRYPOINT can also be used in combination with a script when more than one execution step is required.

Copy the script into the container and run it via ENTRYPOINT on container start.

Example:

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

User can then execute:

$ docker run redis

VOLUME

The VOLUME instruction should be used for any mutable or user-serviceable parts of the image.

VOLUME exposes database storage areas, configuration storage, or files/folders created by the container.

USER

Do not run containers as root. Use USER to change to an non-root user.

Create the user and group as in this example:

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

Avoid installing or using sudo. If you need to, use "gosu" instead.

To minimize the number of layers, avoid switching USER back and forth frequently.

WORKDIR

Always use absolute paths for your WORKDIR.

Use WORKDIR instead of cd commands like "RUN cd … && do-something." They could be hard to read, troubleshoot, and maintain.

ONBUILD

ONBUILD executes on children images derived FROM the current image. ONBUILD can be seen as an instruction the parent Dockerfile gives to the child Dockerfile.

Images built from ONBUILD should get a separate tag, for example: java:8-onbuild or java:9-onbuild.