feat(deploy): add staging environment with dedicated docker compose

- 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

docker-compose.prod.yml

@@ -1,5 +1,3 @@
-version: '3.8'
-
 services:
   # Next.js Application (Production Mode)
   app:
@@ -7,6 +5,8 @@ services:
       context: ./proto
       dockerfile: Dockerfile
       target: runner  # Use runner stage for production (smaller size, no dev dependencies)
+      args:
+        - NEXT_PUBLIC_SITE_URL=${NEXT_PUBLIC_SITE_URL:-https://mozdit.hu}
     container_name: mozdit-app-prod
     ports:
       - "8080:3000" # Host port 8080 elkerüli a lokális npm dev (3000) összeakadást