The CI host builds with the legacy Docker builder (buildx/BuildKit missing on
the runner). Drop the '# syntax' directive and the 'RUN --mount=type=cache'
npm cache mount from the Dockerfile, and remove DOCKER_BUILDKIT=1 from the
workflow. Restores a green build so registry.alwisp.com/jason/mrp-qrcode:latest
publishes and PORT can roll the container off the pinned old digest.