title: docker env variable shows undefined
body: <p>Here's my <code>docker-compose</code></p>
  
  <pre><code>version: '2'
  services:
    weather-cities:
      build:
        context: .
      volumes:
        - .:/usr/app
        - /usr/app/node_modules/
      ports:
        - "8080:8080"
  
      # Set environment variables from this file
      # env_file:
      #   - .env
  
      # Overwrite any env var defined in .env file (if required)
      environment:
        - DB_NAME=test
        - DB_PORT=5432
        - DB_HOST=postgres
        - DB_USERNAME=test
        - DB_PASSWORD=challenge
        - APP_PORT=8080
  
      links:
        - postgres
  
    postgres:
      image: "postgres:9.4.11"
      ports:
        - "5432:5432"
      environment:
        - POSTGRES_USER=test
        - POSTGRES_PASSWORD=challenge
  </code></pre>
  
  <p>Running this migration script</p>
  
  <pre><code>"db:migrate": "./node_modules/.bin/sequelize --migrations-path=migrations --models-path=models --config=config/config.js db:migrate"
  </code></pre>
  
  <p>And here's my <code>Dockerfile</code></p>
  
  <pre><code>FROM node:8.10.0
  
  WORKDIR /usr/app
  
  COPY package.json .
  RUN npm install --quiet
  
  COPY . .
  
  RUN npm run db:migrate
  RUN npm run db:seed
  
  EXPOSE 8080
  
  CMD ["npm", "start"]
  </code></pre>
  
  <p>Output: </p>
  
  <blockquote>
    <p>Loaded configuration file "config/config.js". Using environment
    "development".</p>
    
    <p>ERROR: connect ECONNREFUSED 127.0.0.1:5432</p>
  </blockquote>
  
  <p><code>config.js</code></p>
  
  <pre><code>let config = {
    "development": {
      "username": process.env.DB_USERNAME,
      "password": process.env.DB_PASSWORD,
      "database": process.env.DB_NAME,
      "host": process.env.DB_HOST,
      "dialect": "postgres",
      "port": process.env.DB_PORT
    }
  };
  
  module.exports = config;
  </code></pre>