도커 컴포즈 그대로 해서 docker-compose up -d 명령어 쳤는데, 프론트엔드랑 몽고디비는 up 상태가 되는데, 백엔드에서 몽고디비 커넥션이 안되네요

version: '3'

services:
  frontend:
    image: subicura/guestbook-frontend:latest
    ports:
      - 60000:3000
    environment:
      - PORT=3000
      - GUESTBOOK_API_ADDR=backend:5000
  backend:
    image: subicura/guestbook-backend:latest
    environment:
      - PORT=5000
      - GUESTBOOK_DB_ADDR=mongodb:27017
  mongodb:
    image: mongo:4
    ports:
      - "27017:27017"
    volumes:
      - db_data:/data/db

volumes:
  db_data: {}

에러는 아래와 같이 뜨고 있어요!

backend_1   | Debugger listening on ws://127.0.0.1:9229/b66de091-4ab3-4940-99cf-a5048b8939db

backend_1   | For help see https://nodejs.org/en/docs/inspector

backend_1   | App listening on port 5000

backend_1   | Press Ctrl+C to quit.

backend_1   | Disconnected: unable to reconnect to mongodb://mongodb:27017/guestbook

backend_1   | /backend/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:474

backend_1   |       throw err;

backend_1   |       ^

backend_1   | 

backend_1   | Error: Disconnected: unable to reconnect to mongodb://mongodb:27017/guestbook

backend_1   |     at NativeConnection.db.on (/backend/routes/messages.js:10:11)

backend_1   |     at emitNone (events.js:106:13)

backend_1   |     at NativeConnection.emit (events.js:208:7)

backend_1   |     at NativeConnection.set (/backend/node_modules/mongoose/lib/connection.js:119:12)

backend_1   |     at /backend/node_modules/mongoose/lib/connection.js:529:26

backend_1   |     at err (/backend/node_modules/mongoose/node_modules/mongodb/lib/utils.js:415:14)

backend_1   |     at executeCallback (/backend/node_modules/mongoose/node_modules/mongodb/lib/utils.js:404:25)

backend_1   |     at err (/backend/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:293:21)

backend_1   |     at connectCallback (/backend/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:249:5)

backend_1   |     at process.nextTick (/backend/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:471:7)

backend_1   |     at combinedTickCallback (internal/process/nexttick.js:132:7)

backend_1   |     at process._tickCallback (internal/process/next_tick.js:181:9)