fix: Correct STATIC_URL with leading slash and mount static_volume shared between backend and nginx

docker-compose.yml

@@ -41,6 +41,8 @@ services:
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-shiftplan_secure_password_123}
       POSTGRES_HOST: db
       POSTGRES_PORT: "5432"
+    volumes:
+      - static_volume:/app/staticfiles
     depends_on:
       db:
         condition: service_healthy
@@ -52,6 +54,8 @@ services:
     build:
       context: ./frontend
       dockerfile: Dockerfile
+    volumes:
+      - static_volume:/usr/share/nginx/html/static
     expose:
       - "80"
     depends_on:
@@ -60,3 +64,4 @@ services:
 
 volumes:
   postgres_data:
+  static_volume: