Place the backend Dockerfile at repo root as Dockerfile.backend so
the build context includes .git, giving git describe access for
version stamping. Fix .gitignore pattern to anchor /netwatch-server
so it does not exclude cmd/netwatch-server/. Remove .git from
.dockerignore. Update CI workflow and backend Makefile docker target.
Add .gitea/workflows/check.yml that builds both the root and
backend Docker images on push. Add LICENSE and README.md to the
backend subproject to match repo standards.
