Add a preflight change-detection job to the Docker publish workflow and
derive the build matrix from shared-root and service-specific path
filters. Preserve full publishes on manual dispatch while skipping
unaffected service image builds on routine pushes.
Publish frontend, backend, and remote-gateway images to GHCR on
pushes to main and manual runs. Also update docker-compose to pull
the three service images from ghcr.io instead of Docker Hub.
yinjianm