Add support for ARM64 for built container image

[spwoodcock]

Issue

• Currently the image only builds for AMD64, which is by far the most prevalent architecture.
• ARM64 support for container images becoming more important over time, as infrastructure starts to use a mix of the two.
• As the workflow here: https://github.com/getodk/pyxform-http/blob/master/.github/workflows/publish.yml already uses the docker/build-push-action (which has multi-arch support), it would be nice to also build for ARM64.

Solution

Update the workflow publish.yml to include three extra things:

• QEMU setup stage
• Buildx setup stage
• Add the platform key to the docker/build-push-action, with linux/amd64,linux/arm64 specified.

Full modified workflow:

name: Publish

on: push

env:
    # Use docker.io for Docker Hub if empty
    REGISTRY: ghcr.io
    # github.repository as <account>/<repo>
    IMAGE_NAME: ${{ github.repository }}

jobs:
    build:
        runs-on: ubuntu-latest
        permissions:
            contents: read
            packages: write

        steps:
            - name: Checkout repository
              uses: actions/checkout@v4

            # Login against a Docker registry
            # https://github.com/docker/login-action
            - name: Log into registry ${{ env.REGISTRY }}
              uses: docker/login-action@v3
              with:
                  registry: ${{ env.REGISTRY }}
                  username: ${{ github.actor }}
                  password: ${{ secrets.GITHUB_TOKEN }}

            # Extract metadata (tags, labels) for Docker
            # https://github.com/docker/metadata-action
            - name: Extract Docker metadata
              id: meta
              uses: docker/metadata-action@v5
              with:
                  images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}

            # Setup QEMU emulator to build multi-arch images
            # https://github.com/docker/setup-qemu-action
            - name: Set up QEMU
              uses: docker/setup-qemu-action@v3

            # Configure Buildx for Docker build
            # https://github.com/docker/setup-buildx-action
            - name: Set up Buildx
              uses: docker/setup-buildx-action@v3

            # Build and push Docker image with Buildx
            # https://github.com/docker/build-push-action
            - name: Build and push Docker image
              uses: docker/build-push-action@v5
              with:
                  context: .
                  push: true
                  tags: ${{ steps.meta.outputs.tags }}
                  labels: ${{ steps.meta.outputs.labels }}
                  platforms: 'linux/amd64,linux/arm64'

Additional considerations

• This is a small part of marking ODK Central compatible with ARM64, when providing pre-built images from Build Docker images in GitHub Actions central#546 (users can already build ODK Central ARM64 themselves).
• @lognaturel we already discussed that there is no need for ARM64 support until a user requests it. I actually had a developer wishing to contribute to a tool I maintain that uses pyxform-http as part of the docker-compose stack, using a Macbook M1 for development. They were blocked from doing so by no ARM64 for this image. I don't see much downside to at least having support on pyxform-http only for now 😄

[lognaturel]

Sounds good if you want to make it so, thank you!