- Add docker-compose.staging.yml (app on 127.0.0.1:8081, separate MongoDB
on 127.0.0.1:27019, dedicated volume/network)
- deploy.sh: select compose file and port per environment
(staging 8081, production 8080)
- Dockerfile: accept NEXT_PUBLIC_SITE_URL as build arg (inlined at build
time for NEXT_PUBLIC_ vars); pass it in both compose files
- Add .env.staging.example and .env.production.example