An example of a NodeJS app running on port 4000 along with postgres in to separate containers with Docker Compose.

docker-compose.yml

version: "3"
services:
  app:
    build: .
    environment:
      NODE_ENV: development
    ports:
      - "4000:4000"
    depends_on:
      - postgres
    volumes:
      - ./:/usr/src/app
  postgres:
    image: postgres:11.1-alpine
    environment:
      POSTGRES_DB: betaflag