About

This GitHub Action calculates a hash from the maximal Docker image context. Which can be useful to get a reproducible value for an arbitrary Git tree-ish object.

Hash inputs are taken from git ls-tree. Which means that when any object name or path that is part of the Docker image context changes, the calculated hash changes.

This could be leveraged to e.g.

• Push docker images with tags that can be found in different contexts, across the Git repository as a whole. For example: a release workflow could find an image built by a test workflow.
• Skip unnecessary image builds if there has been no changes to any files included in the maximal Docker image context.

While using e.g. a Git commit hash could work correspondingly it is more sensitive. For example: if contents of a Docker image context hasn't changed between 2 commits, the hash calculation produces identical output.

Note that docker-image-context-hash-action does not calculate the hash of the actual Docker image context.

---

Inputs

• build_context: (optional) The path to the Docker image context. Defaults to the current working directory.

• extra_tree_objects: (optional) Additional Git tree objects, outside of the Docker image context, that should also be included when calculating the hash.

Outputs

hash: The hash of the Docker image context.

Usage

on:
  pull_request:

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Calculate Docker image context hash
        id: context-hash
        uses: 5monkeys/docker-image-context-hash-action@main
        with:
          extra_tree_objects: |
            .dockerignore
            Dockerfile

      - name: Build and push Docker image
        uses: docker/[email protected]
        with:
          context: .
          push: true
          tags: your-image:${{ steps.context-hash.outputs.hash }}

In this example, the docker-image-context-hash-action is used to calculate the hash of the current path as Docker image context, and the resulting hash is used as part of the Docker image tag when building and pushing the image.

The files .dockerignore and Dockerfile are added as extra Git tree objects since they are implicitly excluded from the image context by Docker. Having them as extra Git tree objects results in tracking changes to them identically to files included in the image context.

Note that the docker/build-push-action is used in the example to build and push the Docker image, but this can be replaced with any other action or script that builds and pushes Docker images.

Development

• Install deps: npm ci
• Run tests: npm run test
• Run lint: npm run lint
• Package application: npm run package. Remember to run this before committing anything.