Add multi-architecture (amd64/arm64) container builds with GHCR support

[chrisns]

Summary

This PR adds automated multi-architecture Docker image builds (linux/amd64 and linux/arm64) with automatic publishing to GitHub Container Registry (GHCR).

What's included

• New workflow: .github/workflows/build-push.yml - Builds and pushes multi-arch images on every push to php* branches
• Updated README: Documents the new automated build process alongside the existing manual Docker Hub workflow

Benefits

🚫 No rate limiting
Unlike Docker Hub, GHCR has no anonymous pull rate limits. This is particularly valuable for CI/CD pipelines where Docker Hub's 100 pulls/6 hours limit can cause build failures during busy periods.

💰 Cost savings with ARM support
ARM64 instances (AWS Graviton, Azure Ampere, GCP Tau T2A) are typically 20-40% cheaper than equivalent x86 instances while offering comparable or better performance. Adding arm64 support enables the community to run LocalGov Drupal on these more cost-effective platforms.

🔄 Dynamic tagging
The workflow automatically extracts the base image tag from the Dockerfile, so when you update FROM php:8.4-apache, it will automatically tag as php-8.4-apache - no manual tag updates needed.

📦 Works for upstream and forks
Uses ${{ github.repository }} so the same workflow works in the main repo and any forks without modification.

Image locations

Once merged, images will be available at:

ghcr.io/localgovdrupal/drupal-container:php-8.3-apache

Adding Docker Hub publishing (optional)

If you'd also like to automatically push to Docker Hub alongside GHCR, you can add these steps to the workflow:

      - name: Login to Docker Hub
        uses: docker/login-action@v3
        with:
          username: ${{ secrets.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}

And update the tags in the build step:

          tags: |
            ghcr.io/${{ github.repository }}:${{ steps.extract-tag.outputs.tag }}
            localgovdrupal/apache-php:${{ github.ref_name }}

This would require adding DOCKERHUB_USERNAME and DOCKERHUB_TOKEN secrets to the repository.

---

We've verified the workflow works - our fork's build completed successfully and the multi-arch image is functioning correctly:

$ docker buildx imagetools inspect ghcr.io/co-cddo/drupal-container:php-8.3-apache
Manifests: 
  Platform: linux/amd64
  Platform: linux/arm64

We're happy to make any changes you'd suggest - please let us know if you have any feedback or would like us to adjust anything.