feat: add Docker support with Dockerfile, compose, CI, and oneliner install

- Multi-stage Dockerfile (node build + nginx:alpine serve)
- nginx.conf with SPA fallback, gzip, asset caching
- docker-compose.yml for easy deployment
- GitHub Actions workflow to build & push to ghcr.io on every push
- .dockerignore to keep image lean
- Updated README with Docker-first quick start and badge

docker-compose.yml

@@ -0,0 +1,8 @@
+services:
+  pinchchat:
+    image: ghcr.io/marlburrow/pinchchat:latest
+    # Or build locally:
+    # build: .
+    ports:
+      - "3000:80"
+    restart: unless-stopped