chore(docker): add restart policy to prod services

Add 'restart: unless-stopped' to frontend and backend services
so containers automatically restart after system reboot or crash.

docker/prod/docker-compose.yml

@@ -6,6 +6,7 @@ services:
   frontend:
     container_name: image-uploader-frontend
     image: gitea.lan.hobbyhimmel.de/hobbyhimmel/image-uploader-frontend:latest
+    restart: unless-stopped
     ports:
       - "80:80"
     build: 
@@ -25,6 +26,7 @@ services:
   backend:
     container_name: image-uploader-backend
     image: gitea.lan.hobbyhimmel.de/hobbyhimmel/image-uploader-backend:latest
+    restart: unless-stopped
     build: 
       context: ../../
       dockerfile: docker/prod/backend/Dockerfile