ci: build and push container image on merge to main #742

Summary

Today, container images are only built on GitHub Release publish (release.yml). We need a continuous image build on main so the server can pull pre-built images in later deployment phases.

Goal

• Will add a GitHub Actions workflow triggered on push to main
• It will run after existing CI checks pass.
• Will build image from root Dockerfile
• Push to GHCR:
  • ghcr.io/cameri/nostream:main
  • ghcr.io/cameri/nostream:sha-<commit>
• Will reuse the GHCR login/build/push pattern from release.yml
• And will document any required repo permissions / packages: write