Question from the Docker compose test

What are the errors in the docker-compose.yml file?

version: '3'
services:
  web:
    build: ./web
    ports:
      - "8080:80"
    networks:
      - frontend
    depends_on:
      - app
  app:
    build: ./app
    ports:
      - "8081:80"
    networks:
      - backend
    volumes:
      - app-data:/var/lib/app-data
    environment:
      - APP_MODE=production
networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge
volumes:
  app-data: