Containerize for independent deployment

Adds a Dockerfile, nginx config, docker-compose.yml, and a Gitea Actions
workflow (mirroring goonk's), so rack builds and ships as its own image
rather than being embedded in goonk-cv's build - deploys stay fully
decoupled. Targets a subdomain, not a goonk.se subpath, to avoid needing
path-prefix rewriting kept in sync across the build, edge proxy, and
container.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,2 @@
+# Copy to .env (or set in your shell) to override docker-compose.yml defaults
+HOST_PORT=5677