title: build contains unsupported option: 'ports'
body: <p>Trying to use docker-compose for the first time, but not having much luck. I have the following setup:
  docker-compose version 1.8.0, build f3628c7</p>
  
  <ul>
  <li>/home/GabeThermComposer contains the docker-compose.yml</li>
  <li>/home/GabeThermComposer/GabeThermApache contains Dockerfile</li>
  <li>/home/GabeThermComposer/GabeThermPHPMyAdmin contains Dockerfile</li>
  <li>/home/GabeThermComposer/GabeThermDB contains Dockerfile and nest-init.sql</li>
  </ul>
  
  <p>When I create docker images using the Dockerfile in each subdir, it all works without issues. I was hoping with the docker-compose.yml to do all the seperate building of images at once.</p>
  
  <p>The docker-compose.yml looks like this:</p>
  
  <pre><code>version: '2'
  services:
    GabeThermDB:
      build:
        context: ./GabeThermDB
        dockerfile: Dockerfile
  
    GabeThermApache:
      build:
        context: ./GabeThermApache
        dockerfile: Dockerfile
        ports:
         - "80:80"
  
    GabeThermPHPMyAdmin:
      build:
        context: ./GabeThermPHPMyAdmin
        dockerfile: Dockerfile
        ports:
         - "8080:80"
  </code></pre>
  
  <p>When trying to run "docker-compose up", I get the following error:</p>
  
  <pre><code>ERROR: The Compose file './docker-compose.yml' is invalid because:
  services.GabeThermPHPMyAdmin.build contains unsupported option: 'ports'
  services.GabeThermApache.build contains unsupported option: 'ports'
  </code></pre>
  
  <p>I have no clue on what is wrong with this. I think I did exactly as other examples have shown. Btw, I do know that the "context:" and "dockerfile:" is overdone, but since I'm new, I wanted to be sure to what files I'm pointing in case I forget it automatically dives into the subdir and runs the Dockerfile. </p>
  
  <p>Any help is appreciated.</p>