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oci-build task

A Concourse task for building OCI images. Currently uses buildkit for building.

A stretch goal of this is to support running without privileged: true, though it currently still requires it.

usage

The task implementation is available as an image on Docker Hub at vito/oci-build-task. (This image is built from Dockerfile using the oci-build task itself.)

This task implementation started as a spike to explore patterns around reusable tasks to hopefully lead to a proper RFC. Until that RFC is written and implemented, configuration is still done by way of providing your own task config as follows:

image_resource

First, your task needs to point to the oci-build-task image:

image_resource:
  type: registry-image
  source:
    repository: vito/oci-build-task

params

Next, any of the following optional parameters may be specified:

  • $CONTEXT (default .): the path to the directory to provide as the context for the build.

  • $DOCKERFILE (default $CONTEXT/Dockerfile): the path to the Dockerfile to build.

  • $BUILD_ARG_*: params prefixed with BUILD_ARG_ will be provided as build args. For example BUILD_ARG_foo=bar, will set the foo build arg as bar.

  • $BUILD_ARGS_FILE (default empty): path to a file containing build args in the form foo=bar, one per line. Empty lines are skipped.

    Example file contents:

    [email protected]
HOW_MANY_THINGS=1
DO_THING=false

  • $TARGET (default empty): a target build stage to build.

  • $TARGET_FILE (default empty): path to a file containing the name of the target build stage to build.

  • $REGISTRY_MIRRORS (default empty): registry mirrors to use for docker.io.

  • $UNPACK_ROOTFS (default false): unpack the image as Concourse's image format (rootfs/, metadata.json) for use with the image task step option.

Note: this is the main pain point with reusable tasks - env vars are kind of an awkward way to configure a task. Once the RFC lands these will turn into a JSON structure similar to configuring params on a resource, and task params will become env instead.

inputs

There are no required inputs - your task should just list each artifact it needs as an input. Typically this is in close correlation with $CONTEXT:

params:
  CONTEXT: my-image

inputs:
- name: my-image

Should your build be dependent on multiple inputs, you may want to leave $CONTEXT as its default (.) and set an explicit path to the $DOCKERFILE:

params:
  DOCKERFILE: my-repo/Dockerfile

inputs:
- name: my-repo
- name: some-dependency

It might also make sense to place one input under another, like so:

params:
  CONTEXT: my-repo

inputs:
- name: my-repo
- name: some-dependency
  path: my-repo/some-dependency

Or, to fully rely on the default behavior and use path to wire up the context accordingly, you could set your primary context as path: . and set up any additional inputs underneath:

inputs:
- name: my-repo
  path: .
- name: some-dependency

outputs

A single output named image may be configured:

outputs:
- name: image

The output will contain the following files:

  • image.tar: the OCI image tarball. This tarball can be uploaded to a registry using the Registry Image resource.

  • digest: the digest of the OCI config. This file can be used to tag the image after it has been loaded with docker load, like so:

    docker load -i image/image.tar
docker tag $(cat image/digest) my-name

If $UNPACK_ROOTFS is configured, the following additional entries will be created:

  • rootfs/*: the unpacked contents of the image's filesystem.

  • metadata.json: a JSON file containing the image's env and user configuration.

This is a Concourse-specific format to support using the newly built image for a subsequent task by pointing the task step's image option to the output, like so:

plan:
- task: build-image
  output_mapping: {image: my-built-image}
- task: use-image
  image: my-built-image

(The output_mapping here is just for clarity; alternatively you could just set image: image.)

Note: at some point Concourse will likely standardize on OCI instead.

caches

Caching can be enabled by caching the cache path on the task:

caches:
- path: cache

run

Your task should run the build executable:

run:
  path: build

migrating from the docker-image resource

The docker-image resource was previously used for building and pushing a Docker image to a registry in one fell swoop.

The oci-build task, in contrast, only supports building images - it does not support pushing or even tagging the image. It can be used to build an image and use it for a subsequent task image without pushing it to a registry, by configuring $UNPACK_ROOTFS.

In order to push the newly built image, you can use a resource like the registry-image resource like so:

resources:
- name: my-image-src
  type: git
  source:
    uri: https://github.com/...

- name: my-image
  type: registry-image
  source:
    repository: my-user/my-repo

jobs:
- name: build-and-push
  plan:
  # fetch repository source (containing Dockerfile)
  - get: my-image-src

  # build using `oci-build` task
  #
  # note: this task config could be pushed into `my-image-src` and loaded using
  # `file:` instead
  - task: build
    privileged: true
    config:
      platform: linux

      image_resource:
        type: registry-image
        source:
          repository: vito/oci-build-task

      inputs:
      - name: my-image-src
        path: .

      outputs:
      - name: image

      run:
        path: build

  # push using `registry-image` resource
  - put: my-image
    params: {image: image/image.tar}

differences from builder task

The builder task was a stepping stone that led to the oci-build task. It is now deprecated. The transition should be relatively smooth, with the following differences:

  • The oci-build task does not support configuring $REPOSITORY or $TAG.
    • for running the image with docker, a digest file is provided which can be tagged with docker tag
    • for pushing the image, the repository and tag are configured in the registry-image resource
  • The oci-build task has a more efficient caching implementation. By using buildkit directly we can make use of its local cache exporter/importer, which doesn't require a separate translation step for saving into the task cache.
  • This task is written in Go instead of Bash, and has tests!

example

This repo contains an example.yml, which builds the image for the task itself:

fly -t dev execute -c example.yml -o image=. -p
docker load -i image.tar

That -p at the end is not a typo; it runs the task with elevated privileges.