chore: add local docker-compose.yml for development

Mirrors the production frontend service from docker-compose.prod.yml
but skips Caddy/SSL and exposes Nginx directly on host port 8080.

https://claude.ai/code/session_01MZiV8aacCJbEKFLJkdRPZE

docker-compose.yml

@@ -0,0 +1,6 @@
+services:
+  frontend:
+    build: .
+    restart: unless-stopped
+    ports:
+      - "8080:80"