docker-compose.yml

version: "3.8"
services:
  frontend:
    build: frontend
    ports:
      - 8080:8080
  backend:
    build: backend
    environment:
      MONGODB_CONNECTION_STRING: "mongodb://mongodb/twitter-clone" # error here: mongodb's port is forwared to 37017. Either change it here or forward it to 21017
    ports:
      - 3000:3000
  mongodb:
    image: mongo:4.2
    ports:
      - 27017:27017 # changed 31017 to 21017
    volumes:
      - ./data:/data/db