I have a setup like this in my production compose.yml file.

services:
   
  proxy:
    container_name: app-proxy
    image: nginx:1.25.5
    ports:
      - 80:80
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
    depends_on:
      - frontend
    networks:
      - app-net

  frontend:
    container_name: frontend
    image: my-org/nextjs-frontend:prod
    build:
      context: ./frontend
      dockerfile: .docker/prod/Dockerfile
    command: ["node", "server.js"]
    depends_on:
      - backend
    networks:
      - app-net

  backend:
    container_name: app-backend
    image: my-org/wagtail-backend:prod
    build:
      context: ./backend
      dockerfile: Dockerfile
    environment:
      - DB_NAME=${DB_NAME}
      - DB_USER=${DB_USER}
      - DB_PASSWORD=${DB_PASSWORD}
      - DEBUG=False
      - CSRF_TRUSTED_ORIGINS=${CSRF_TRUSTED_ORIGINS}
    volumes:
      - backend-static:/staticfiles
      - backend-media:/mediafiles
    command: >
      sh -c "
      set -xe;
      python manage.py migrate --noinput;
      gunicorn --bind 0.0.0.0:8000 backend.wsgi:application
      "
    depends_on:
      - db
    networks:
      - app-net
 
  db:
    container_name: app-db
    image: postgres:16.3
    environment:
      - POSTGRES_DB=${DB_NAME}
      - POSTGRES_USER=${DB_USER}
      - POSTGRES_PASSWORD=${DB_PASSWORD}
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - app-net

volumes:
  backend-static:
  backend-media:
  db-data:

networks:
  app-net:
    driver: bridge

What I did was start the db and backend services with docker compose up backend -d, ensuring they were up and running. I then proceeded to try building the image for the frontend (which is a Next.js app) with docker build frontend, because the frontend needs to fetch data from the backend (up and running blissfully) for pre-rendering pages during its build process with yarn (specified in its Dockerfile).The error I got from this build was a network error stating that it can’t resolve the backend service. Here’s the error message:

32.83       at node:internal/deps/undici/undici:12618:11
32.83       at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
32.83     cause: Error: getaddrinfo EAI_AGAIN backend
32.83         at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:107:26)
32.83         at GetAddrInfoReqWrap.callbackTrampoline (node:internal/async_hooks:128:17) {
32.83       errno: -3001,
32.83       code: 'EAI_AGAIN',
32.83       syscall: 'getaddrinfo',
32.83       hostname: 'backend'
32.83     }

Can anyone tell me why I get this error? Is the backend service exposed to this build process?

The fetch for the backend service is correctly defined in my nextjs codebase because I use the same setup for dev (except without SSG using yarn build).