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Ploutos: Docker packaging

Contents:

Known issues

Terminology

Docker terminology regarding the location/identity of an image published to a registry (let's assume Docker Hub) is a bit confusing. Dockers' own official documentation conflates the terms "image" and "tag". When configuring the Ploutos workflow we therefore use the following terminology:

# Using Docker Hub terminology, for a Docker image named nlnetlabs/krill:v0.1.2-arm64:
#   - The Organization would be 'nlnetlabs'.
#   - The Repository would be 'krill'.
#   - The Tag would be v0.1.2-arm64
# Collectively I refer to the combination of <org>/<repo>:<tag> as the 'image' name,

Source: https://github.com/NLnetLabs/.github/blob/main/.github/workflows/pkg-rust.yml

Inputs

Workflow inputs

Input Type Required Description
docker_org string Yes E.g. nlnetlabs.
docker_repo string Yes E.g. krill.
docker_build_rules matrix Yes See below.
docker_sanity_check_command string No A command to pass to docker run. If it returns a non-zero exit code it will cause the packaging workflow to fail. The command is intended to be a simple sanity check of the built image and should return quickly. It will only be run against images built for the x86_64 architecture as in order for docker run to work the image CPU architecture must match the host runner CPU architecture. As such when building images for non-x86_64 architectures it does NOT verify that ALL built images are sane.
docker_file_path string No The path relative to the Git checkout to the Dockerfile. Defaults to ./Dockerfile.
docker_context_path string No The path relative to the Git checkout to use as the Docker context. Defaults to ..

Note: There is no input for specifying the Docker tag because the tag is automatically determined based on the current Git branch/tag and architecture "shortname" (taken from the docker_build_rules matrix).

Docker build rules

A rules matrix with the following keys must be provided to guide the build process:

Matrix Key Required Description
platform Yes Set the target platform for the build, e.g. linux/amd64. See 1, 2 and 3.
shortname Yes Suffixes the tag of the architecture specific "manifest" image with this value, e.g. amd64.
target No Used to determine the correct cross-compiled binary GitHub Actions artifact to compile & download. Only used when mode is copy.
mode No copy (for cross-compiled targets) or build. Passed through to the Dockerfile as build arg MODE.
cargo_args No Can be used when testing, e.g. set to --no-default-features to speed up the application build. Passed through to the Dockerfile as build arg CARGO_ARGS.

Example using an inline YAML string matrix definition:

jobs:
  my_pkg_job:
    uses: NLnetLabs/ploutos/.github/workflows/pkg-rust.yml@v5
    with:
      docker_build_rules: |
        include:
          - platform: "linux/amd64"
            shortname: "amd64"
            mode: "build"

          - platform: "linux/arm/v6"
            shortname: "armv6"
            target: "arm-unknown-linux-musleabihf"
            mode: "copy"

Dockerfile build arguments

The Ploutos workflow will invoke docker buildx passing --build-arg <varname>=<value> for the following custom build arguments.

The Docker context will be the root of a clone of the callers GitHub repository.

Your Dockerfile MUST define corresponding ARG <varname>[=<default value>] instructions for these build arguments.

Build Arg Description
MODE=build The Dockerfile should build the application from sources available in the Docker context.
MODE=copy The pre-compiled binaries will be made available to the build process in subdirectory dockerbin/$TARGETPLATFORM/* of the Docker build context, where $TARGETPLATFORM is a variable made available to the Dockerfile build process by Docker, e.g. linux/amd64. For an example see https://github.com/NLnetLabs/routinator/blob/v0.11.3/Dockerfile#L99.
CARGO_ARGS=... Only relevant when MODE is build. Expected to be passed to the Cargo build process, e.g. cargo build ... ${CARGO_ARGS} or cargo install ... ${CARGO_ARGS}. For an example see https://github.com/NLnetLabs/routinator/blob/v0.11.3/Dockerfile#L92.

Docker Hub secrets

The Ploutos workflow supports two Docker specific secrets which can be passed to the workflow like so:

jobs:
  full:
    uses: NLnetLabs/ploutos/.github/workflows/pkg-rust.yml@v5
    secrets:
      DOCKER_HUB_ID: ${{ secrets.YOUR_DOCKER_HUB_ID }}
      DOCKER_HUB_TOKEN: ${{ secrets.YOUR_DOCKER_HUB_TOKEN }}

Or, if you are willing to trust the packaging workflow with all of your secrets!!!

jobs:
  full:
    uses: NLnetLabs/ploutos/.github/workflows/pkg-rust.yml@v5
    secrets: inherit

If either of the DOCKER_HUB_ID and/or DOCKER_HUB_TOKEN secrets are defined, the workflow prepare job will attempt to login to Docker Hub using these credentials and will fail the Ploutos workflow if it is unable to login.

Best practice is to use a separately created Docker Hub access token for automation purposes that has minimal access rights. The Ploutos workflow needs write access but not delete access.

Note: If neither of the DOCKER_HUB_ID and DOCKER_HUB_TOKEN secrets are defined then the Ploutos workflow will NOT atttempt to publish images to Docker Hub.

Outputs

A GitHub Actions artifact will be attached to the workflow run with the name tmp-docker-image-<shortname>. The artifact will be a zip file, inside which will be a tar file called docker-<shortname>-img.tar. The tar file is the output of the docker save command and can be loaded into a local Docker daemon using the docker load command.

If the required secrets are defined (see above), and the git ref is either the main branch or a v* tag, then the Docker image will be published to Docker Hub with the generated image name (see above).

How it works

The docker workflow job will do a Git checkout of the repository that hosts the caller workflow.

Building

When using the cross job to cross-compile your application for different architectures you do not want to build the application again when building the Docker image from the Dockerfile.

You can direct the Ploutos workflow to use pre-cross-compiled binaries by setting the mode to copy instead of the default build in your docker_build_rules(_path) input matrix.

You must however make sure that your Dockerfile supports the build arguments that the Ploutos workflow will pass to it (see above).

Publication

The Ploutos workflow is able to output built Docker images in three ways:

  1. Output Docker images as GitHub Actions artifacts attached to the workflow run This can be useful for testing or manual distribution or if you don't (yet) have a Docker Hub login and/or access token.

  2. Publish Docker images to Docker Hub: For the common single architecture case this is what you probably want.

  3. Publish multi-arch Docker images AND a Docker manifest to Docker Hub: This is useful when publishing the same image for multiple architectures to enable the end user to run the image without needing to specify the desired architecture.

Generated image names

As stated above there is no way to manually control the tag given to the created Docker images. The images need to have distinct tags per architecture and per version/release type. For these reasons the workflow determines the tag itself. Possible tags that the workflow can generate are:

Image Name Archtecture Specific Tag Multi-Arch Tag Conditions
<docker_org>/<docker_repo> :vX.Y.Z-<shortname> :vX.Y.Z No dash - in git ref
<docker_org>/<docker_repo> :unstable-<shortname> :unstable Branch is main
<docker_org>/<docker_repo> :latest-<shortname> :latest No dash - in git ref and not main
<docker_org>/<docker_repo> :test-<shortname> :test Neither main nor vX.Y.Z tag

Footnotes

  1. https://go.dev/doc/install/source#environment (from 4) ↩

  2. https://github.com/containerd/containerd/blob/v1.4.3/platforms/database.go#L83 ↩

  3. https://stackoverflow.com/a/70889505 ↩

  4. https://github.com/docker-library/official-images#architectures-other-than-amd64 (from 5) ↩

  5. https://docs.docker.com/desktop/multi-arch/ ↩