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>

docker-compose.yml

@@ -0,0 +1,8 @@
+services:
+  web:
+    image: repo.explewd.com/explewd/rack:latest
+    container_name: rack-web
+    restart: unless-stopped
+    ports:
+      - "${HOST_PORT:-5677}:80"
+    pull_policy: always