Add test.yml workflow for Docker test builds

[techroy23]

Add GitHub Actions workflow for test branch builds

• Introduce .github/workflows/test.yml to handle Docker image builds from the test branch
• Adjust triggers to run on push to test and manual dispatch
• Push images with test-specific tags (test-latest, test-) to GHCR and Docker Hub
• Ensure commits and pushes target the test branch instead of main
• Keep production workflow untouched while isolating experimental builds