fix: resolve Docker multi-registry deployment issues

[philprime]

Fix Docker Multi-Registry Deployment

This PR resolves issues with Docker multi-registry multi-arch builds that were failing in the release workflow.

Problem

The Docker build workflow was failing with errors when attempting to push images by digest to multiple registries simultaneously:

ERROR: failed to push docker.io/techprimate/github-actions-utils-cli:latest by digest

Root Cause

Docker BuildKit's push-by-digest=true mode works with a single canonical registry. Attempting to push the same digest to multiple registries at once is not supported because:

1. Each registry maintains its own blob storage
2. Digests are registry-specific references
3. A single build can only push by digest to one location

Solution

Implemented the recommended approach for multi-registry multi-arch builds:

Build Step (Per Platform)

• ✅ Build platform-specific image (amd64 or arm64)
• ✅ Push by digest to GHCR only (primary registry)
• ✅ Export digest for merge step
• ⚡ Runs in parallel for each platform

Merge Step (Single Job)

• ✅ Download all platform digests
• ✅ Reference GHCR digests to create multi-arch manifest
• ✅ Apply tags for both GHCR and Docker Hub in one command
• ✅ BuildKit automatically copies layers to Docker Hub

Changes

.github/workflows/build-cli-docker.yml

• Removed docker.io from digest-based push (build step now pushes only to GHCR)
• Simplified build step by removing conflicting metadata-action usage
• Updated merge step to reference GHCR digests and tag both registries
• Kept OCI-compliant labels for proper image metadata

.github/workflows/release.yml

• Added fix-release branch to triggers for testing (can be removed after merge)

Testing

✅ Successfully tested on fix-release branch: https://github.com/techprimate/github-actions-utils-cli/actions/runs/19237095622

Results:

• ✅ Multi-arch builds completed (linux/amd64, linux/arm64)
• ✅ Images published to GHCR
• ✅ Images published to Docker Hub
• ✅ Manifest list created successfully
• ✅ Latest tag applied to both registries

Docker Images

After this PR, images will be available at:

• 📦 GHCR: ghcr.io/techprimate/github-actions-utils-cli:latest
• 🐳 Docker Hub: docker.io/techprimate/github-actions-utils-cli:latest

Both support:

• linux/amd64
• linux/arm64

Verification

Pull and test the images:

# From GHCR
docker pull ghcr.io/techprimate/github-actions-utils-cli:latest
docker run --rm ghcr.io/techprimate/github-actions-utils-cli:latest --version

# From Docker Hub
docker pull docker.io/techprimate/github-actions-utils-cli:latest
docker run --rm docker.io/techprimate/github-actions-utils-cli:latest --version

Follow-Up

After merging, optionally remove the fix-release branch trigger from release.yml if no longer needed for testing.