Add GitHub Release creation to tag workflow

[JulianKahnert]

Summary

• Adds a create-release job to docker-tag.yaml that runs in parallel with the existing Docker build when a version tag is pushed
• Generates release notes via gh api .../releases/generate-notes (same approach as HeadsUpKit)
• Determines the previous tag automatically for a proper diff range
• Creates a GitHub Release with the generated notes via softprops/action-gh-release@v2

Test plan

• Push a version tag and verify a GitHub Release is created with auto-generated notes
• Docker image build continues to work as before