title: Passing arguments for Dockerfiles using Docker compose
body: <p>I'm trying to use parametrize my dockerfiles on build phase and use arguments in Docker-compose. For example in Docker compose I have defined one service called bpp as following:</p>
  
  <pre><code>bpp:
    build:
      context: .
      dockerfile: Dockerfile.bpp
      args:
        gp : 8080
    image: serv/bpp
    restart: always
    depends_on:
      - data
    links:
      - data
  </code></pre>
  
  <p>I'm trying to pass argument named gp to Dockerfile.bpp, where I'm using argument when starting a Python application, exposing a port etc.
  For example in dockerfile.bpp in trying to expose port gp as following:</p>
  
  <pre><code>EXPOSE gp
  </code></pre>
  
  <p>However, when building docker file by command <em>docker-compose build</em> I get following error:
  ERROR: Service 'bpp' failed to build: Invalid containerPort: gp</p>
  
  <p>It seems that argument gp is not visible in dockerfile. Any suggestions?</p>