Unable to use default github actions env vars for composite step - github-actions

Question

I'm trying to utilize some of the default github actions env vars within my composite github action step. I recall reading (the link is long lost) somewhere that composite steps must be passed all of their parameters and don't have access to all the env vars, so I'm trying to do that. However, I can't get the env var value to successfully get passed.
Notice here I'm using 3 different forms of syntax:
- name: Do a thing
uses: ./.github/actions/my-action
with:
repositoryUrl: ${{ env.GITHUB_SERVER_URL }}/$GITHUB_REPOSITORY
commitSha: ${GITHUB_SHA}
context: ${{ env.DOCKER_CONTEXT_PATH }}
tags: ${{ needs.generate-tag.outputs.DOCKER_IMAGE }}
dockerfile: ${{ env.DOCKERFILE_PATH }}
push: true
platforms: linux/amd64, linux/arm64
Those env vars get used by my action, and they're all read as a normal string except for GITHUB_SERVER_URL which is empty. I know this because I can see this snippet in the build output:
--label org.opencontainers.image.source="/$GITHUB_REPOSITORY" --label org.opencontainers.image.revision="${GITHUB_SHA}"
Then within .github/actions/my-action/action.yml I have:
name: "Build image"
description: "Build and conditionally push an image to a remote ECR registry"
inputs:
repositoryUrl:
description: "The github repository URL"
required: true
commitSha:
description: "The commit sha associated with this image"
required: true
tags:
description: "If a new image is built, it will be assigned all of these tags"
required: true
context:
description: "Docker context to use when building"
required: true
dockerfile:
description: "Path to Dockerfile"
required: true
platforms:
description: "Platforms to build"
required: true
push:
description: "Whether to push the image after it's built"
required: true
default: 'false'
runs:
using: "composite"
steps:
- name: Set up QEMU
uses: docker/setup-qemu-action#v2
- name: Set up Docker Buildx
uses: docker/setup-buildx-action#v2
- name: Build docker image
uses: docker/build-push-action#v3
with:
context: ${{ inputs.context }}
tags: ${{ inputs.tags }}
file: ${{ inputs.dockerfile }}
platforms: ${{ inputs.platforms }}
push: ${{ inputs.push }}
labels: |
org.opencontainers.image.source="${{ inputs.repositoryUrl }}"
org.opencontainers.image.revision="${{ inputs.commitSha }}"
How can I ensure these values are passed dynamically without having to hard code them as strings?

Answer 1

Looks like in order to utilize github's native env vars within a composite step's with: section, you need to use ${{ github.server_url }} instead of any form of GITHUB_SERVER_URL env var. This means my action looks like:
runs:
using: "composite"
steps:
- name: Generate labels
id: labels
shell: bash
run: echo "CREATED=$(date +'%Y-%m-%dT%H:%M:%S')" >> $GITHUB_OUTPUT
- name: Set up QEMU
uses: docker/setup-qemu-action#v2
- name: Set up Docker Buildx
uses: docker/setup-buildx-action#v2
- name: Build docker image
uses: docker/build-push-action#v3
with:
context: ${{ inputs.context }}
tags: ${{ inputs.tags }}
file: ${{ inputs.dockerfile }}
platforms: ${{ inputs.platforms }}
push: ${{ inputs.push }}
labels: |
org.opencontainers.image.source="${{ github.server_url }}/${{ github.repository }}"
org.opencontainers.image.revision="${{ github.sha }}"
org.opencontainers.image.created="${{ steps.labels.outputs.CREATED }}"
ref: https://docs.github.com/en/actions/learn-github-actions/contexts#github-context