GitHub Actions Docker Shim

🐋 Shim that enables using private ghcr.io images in GitHub Actions.

Motivations

Currently, there isn't a good story for authoring GitHub Actions backed by private Docker images. Unlike workflow services and jobs, there is no way to transparently provide registry credentials to image backed Actions. Some potential workarounds include manually authenticating inside a composite action, deploying your own Action runners, or making your image public.

This repository produces a tool which aims to solve this problem as seamlessly as possible.

Usage

Note

These examples assumes that you author an action from a repository named Example/example-action, which produces an image named ghcr.io/example/example-action.

Initial Integration

Modify your action.yml to use the latest release of ghcr.io/joshdk/actions-docker-shim and to add a token input with a default value of ${{ github.token }}.

name: Example
description: An example action, wow!

+inputs:
+  token:
+    description: GitHub Actions workflow token.
+    default: ${{ github.token }}

runs:
  using: docker
-  image: docker://ghcr.io/example/example-action:v1.2.3
+  image: docker://ghcr.io/joshdk/actions-docker-shim:v0.1.0

Modify the caller workflow to grant the packages: read permission so that your ghcr.io image can be pulled.

jobs:
  example:
+    permissions:
+      packages: read

    steps:
      - uses: Example/example-action@v1.2.3

How this works

When GitHub runs a workflow which references your action, the ghcr.io/joshdk/actions-docker-shim image is pulled and run instead of your image.

The shim determines the name of the GitHub repository which hosts this action (Example/example-action) and the ref at which this action was references (v1.2.3). Using this information, the name of this image is speculated to be ghcr.io/example/example-action:v1.2.3.

The shim performs a login to ghcr.io using the provided token input, and then pulls the target image.

Finally, the target image is run on the GitHub runner using the same set of volume mappings, environment variables, & arguments that GitHub would have used to run your image.

Specifying Images

By default, the git tag/branch/sha that is used when referring to your action will be used to determine the image tag to run. You can see here how the image name is derived.

jobs:
  example:
    steps:
      # Will run ghcr.io/example/example-action:v1.2.3
      - uses: Example/example-action@v1.2.3

      # Will run ghcr.io/example/example-action:v1.2
      - uses: Example/example-action@v1.2

      # Will run ghcr.io/example/example-action:v1
      - uses: Example/example-action@v1

      # Will run ghcr.io/example/example-action:master
      - uses: Example/example-action@master
      
      # Will run ghcr.io/example/example-action:a35f...b316
      - uses: Example/example-action@a35f...b316

If you want your action to use a specific image tag, then you can set one manually in action.yml.

runs:
  using: docker
  image: docker://ghcr.io/joshdk/actions-docker-shim:v0.1.0
+  args:
+    - --shim-image-tag=snapshot

If your image isn't named the same as your action repository, that can be overridden as well.

runs:
  using: docker
  image: docker://ghcr.io/joshdk/actions-docker-shim:v0.1.0
  args:
    - --shim-image-tag=snapshot
+    - --shim-image=example/some-other-image

Authentication

If you need to provide a token using a custom input name (e.g. to avoid changing the interface for your action) then you can specify the env var name to use.

name: Example
description: An example action, wow!

inputs:
-  token:
+  custom-token:
    description: GitHub Actions workflow token.
    default: ${{ github.token }}

runs:
  using: docker
  image: docker://ghcr.io/joshdk/actions-docker-shim:v0.1.0
+  args:
+    - --shim-token-env=INPUT_CUSTOM-TOKEN

License

This code is distributed under the MIT License, see LICENSE.txt for more information.