Add Docker Compose deployment with multi-stage build and nginx

Multi-stage Dockerfile builds the Vite app in Node 22 and serves static
assets from nginx:alpine. Includes gzip compression, SPA fallback routing,
immutable cache headers for hashed assets, and configurable port mapping
(default 8080). Deploy with `docker compose up -d`.

.dockerignore

@@ -0,0 +1,6 @@
+node_modules
+.git
+dist
+openspec
+*.md
+.claude