feat: publish an official Docker image (Dockerfile + GHCR publish workflow)

[NitinKumar004]

There is no Dockerfile today. A published image is the biggest reach-expander — it unblocks non-Go users and the #224 standalone-server mode. Depends on / pairs with #224.

Build: a multi-stage Dockerfile building cmd/cloudemu serve (from #224), a GitHub Actions workflow publishing to GHCR on tag, and a docker-compose.yml example.

Acceptance: docker run cloudemu starts the server and a real SDK client (endpoint override) drives it successfully.