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Docker Bump GitHub Action

A GitHub action that can be used to trigger a GitHub Actions workflow or GitHub App webhook whenever a target Docker image is not up-to-date with its dependent base image. The canonical scenario is to trigger a GitHub Actions workflow to run that will rebuild a Docker image whenever its base image has been updated. But you're free to define whatever logic you want to have executed when this out-of-date condition occurs.

This is implemented by using the repository-dispatch GitHub action to create repository_dispatch events. These events can then be responded to by a GitHub Actions workflow or GitHub App webhook.

It sounds complicated but it's actually easy to set up.

Usage

Dispatch an event to the current repository whenever ghcr.io/mthalman/docker-bump-action-example:latest is not up-to-date with alpine:latest.

name: Monitor for Base Image Update

on:
  schedule:
  - cron: "0 2 * * *"

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: mthalman/docker-bump-action@v0
        with:
          target-image-name: ghcr.io/mthalman/docker-bump-action-example:latest
          dockerfile: Dockerfile

Add the following in the workflow which builds your Docker image:

on:
  repository_dispatch:
    types: [base-image-update]

See a working example of this workflow scenario in the docker-bump-action-example repo.

Specifying a Dockerfile is not the only way to configure this. See Examples below for more usage patterns.

Action inputs

Name	Description	Default
target-image-name	Required Name of the image to check.
base-image-name	Name of the base image the target image is based on. Required when dockerfile is not set. See Image Name Derivation.
dockerfile	Path to the Dockerfile from which to derive image names. Required when base-image-name is not set and the target image does not contain a org.opencontainers.image.base.name label/annotation. See Image Name Derivation.
base-stage-name	Name of the stage within the Dockerfile from which to derive the name of the base image. See Image Name Derivation.
arch	Default architecture of the target image. This is used to resolve a multi-arch tag to a specifc image.	amd64
repository	The full name of the repository to send the dispatch.	${{ github.repository }}
event-type	A custom webhook event name.	base-image-update
token	An access token with the appropriate permissions. See Token.	${{ github.token }}

OS Support

Support is limited to Linux container images only.

Token

In order to create the repository_dispatch events, the action requires access to the repo via a token.

The default GITHUB_TOKEN token can only be used if you are dispatching to the same repository that the workflow is executing in.

To dispatch to a separate repository you must create a personal access token (PAT) with the repo scope and store it as a secret. If you will be dispatching to a public repository then you can use the more limited public_repo scope.

You can also use a fine-grained personal access token (beta). It needs the following permissions on the target repositories:

• contents: read & write
• metadata: read only (automatically selected when selecting the contents permission)

Base Image Name Derivation

The base image name can either be provided explicitly via the base-image-name input or can be derived in one of two ways:

• the content of the Dockerfile specified by the dockerfile input.
• the target image contains org.opencontainers.image.base.name as a label or annotation.

Be sure to examine the log output from the action to verify which image name it is using.

Derivation from Dockerfile

Depending on how you've structured your Dockerfiles (specifically for multi-stage Dockerfiles), the base image name that is derived by the algorithm may not be what you intended. If the base image name is not what you intended, you can override it via the base-image-name or base-stage-name inputs.

The algorithm for deriving the image names is described below:

1. Find the last stage defined in the Dockerfile.
2. Starting from the last stage, walk the stage hierarchy until the root stage is found.
3. The image name of the root stage is considered the base image name.

Dispatch Payload

When the repository dispatch occurs, a payload is sent along with it. This payload includes metadata describing the state of the image that resulted in an update being needed. This payload can be retrieved in the workflow that responds to the dispatch via the client-payload event. Consuming this payload is completely optional and only necessary if your workflow requires more context regarding the dispatch.

name: Build Image
on:
  repository_dispatch:
    types: [base-image-update]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - name: Show Rebuild Info
      if: ${{ github.event.client_payload }}
      run: |
        target="${{ github.event.client_payload.updates[0].targetImageName }}"
        base="${{ github.event.client_payload.updates[0].baseImageName }}"
        echo "Rebuilding $target to be in sync with $base"

Payload Schema

Each object in the updates array represents an image that the action has determined requires an update. Currently the action only supports targeting single images and so this array will always have a single element. See #3 for support for multiple images.

{
  "updates": [
    {
      "targetImageName": "Name of the target image provided as input to the action",
      "targetImageDigest": "Current digest of the target image",
      "dockerfile": "Relative path of the Dockerfile",
      "baseImageName": "Name of the base image that was either provided as input to the action or derived via other state",
      "baseImageDigest": "Current digest of the base image"
    }
  ]
}

Examples

Base Name Label/Annotation

If the dockerfile input is not provided, the action attempts to query the target image for the org.opencontainers.image.base.name label or annotation. If set, it will use that value as the base image name to check against.

This example Dockerfile shows how the label could be set.

Note

Alternatively, labels and annotations can be set through various means when pushing an image to a registry:

• build-push-action labels
• build-push-action annotations
• BuildKit annotations

FROM alpine:latest
LABEL org.opencontainers.image.base.name=alpine:latest

The action is configured to only specify the target image name. That's all that's needed in order to determine the base image name.

- uses: mthalman/docker-bump-action@v0
  with:
    target-image-name: ghcr.io/mthalman/docker-bump-action-example:latest

See a working example of this workflow scenario in the docker-bump-action-example repo.

Explicitly set base stage name

In this example, the test stage is the last stage listed. Based on the algorithm described above, the action would start at the last stage and walk the hierarchy until it reached the root stage, which has an image name of mcr.microsoft.com/dotnet/sdk:latest. That's what would be used as the base image name. However, that's not what we want since the actually shipping app image is based on mcr.microsoft.com/dotnet/runtime:latest.

# Build image
FROM mcr.microsoft.com/dotnet/sdk:latest AS build
<content>

# App image
FROM mcr.microsoft.com/dotnet/runtime:latest AS app
COPY --from=build /app /app

# Test image
FROM build AS test
<content>

To configure the action to use mcr.microsoft.com/dotnet/runtime:latest as the base image name, the base-stage-name input is set to app.

- uses: mthalman/docker-bump-action@v0
  with:
    target-image-name: ghcr.io/mthalman/docker-bump-action-example:latest
    base-stage-name: app

Indicating the base stage name rather than the base image name can be more convenient because it allows the image name in the Dockerfile to be updated without needing to also change the workflow file.

See a working example of this workflow scenario in the docker-bump-action-example repo.

Explicitly set base image name

In this example, the base image name used in the Dockerfile is dynamic so the action can't use the Dockerfile as input.

ARG BASE_IMAGE
FROM $BASE_IMAGE
<rest of content>

Instead, the desired base image name is explicitly provided as input. Here, the base image name is explicitly set to alpine:latest.

- uses: mthalman/docker-bump-action@v0
  with:
    target-image-name: ghcr.io/mthalman/docker-bump-action-example:latest
    base-image-name: alpine:latest

See a working example of this workflow scenario in the docker-bump-action-example repo.

Troubleshooting

Error: Resource not accessible by integration

This error occurs when using the GITHUB_TOKEN token (the default token value) and not having write permissions for the repo. Go to your GitHub repo's Settings page at https://github.com/<owner>/<repo>/settings/actions and ensure that Read and write permissions is set for Workflow permissions.

Error: Repository not found, OR token has insufficient permissions.

This error occurs when the personal access token (PAT) is not configured with appropriate permissions. See Token for configuration details.