Hierarchy Folder
test_dock4
- db
- web
  - node_modules
  - Dockerfile
  - index.js
  - package-lock.json
  - package.json
- docker-compose.yml

//=================================
Dockerfile
FROM node:8.12.0
# Create app directory
RUN mkdir /webapp
WORKDIR /webapp

# Install app dependencies
COPY package.json /webapp
RUN npm install

# Bundle app source
COPY . /webapp

EXPOSE 8080

CMD ["node", "index.js"]

//=================================
docker-compose.yml
version: '2'
services:
  web:
    container_name: web
    build: ./web/
    links:
      - db
    environment:
      - DATABASE_URL=postgres://api:123@db:5432/new_databases
    command: node index.js
    ports:
      - "8080:8080"
    volumes:
      - ./web:/webapp
  db:
    image: postgres
    container_name: db
    ports:
      - "5432:5432"
    environment:
      - POSTGRES_USER=api
      - POSTGRES_PASSWORD=123
      - POSTGRES_DATABASE=new_databases

//=================================
index.js
const express = require('express');
const app = express();

const pg = require('pg');
const PORT = 8080;

app.get('/', (req, res) => {
    res.status(200).json({msg: 'Testing Ok Bos'})
});

app.listen(PORT, () => {
    console.log(`Server is listening on port ${PORT}`)
});

//=================================
Error:
web    |     Error: Cannot find module '/webapp/index.js'
web    |     at Function.Module._resolveFilename (module.js:548:15)
web    |     at Function.Module._load (module.js:475:25)
web    |     at Function.Module.runMain (module.js:694:10)
web    |     at startup (bootstrap_node.js:204:16)
web    |     at bootstrap_node.js:625:3
web exited with code 1