title: Override specific docker-compose values with no value via file composition
body: <p>In my project I have a series of docker-compose yml files. I run my application as such <code>docker-compose -f docker-compose.yml -f docker-compose.ci.yml up -d</code>.</p>
  
  <p>docker-compose.yml</p>
  
  <pre><code>db:
      container_name: db
      image: mysql
      ...
  ui:
      container_name: ui
      image: flask
  </code></pre>
  
  <p>docker-compose.ci.yml</p>
  
  <pre><code>db:
      image: db:$BUILD_NUM
  ui:
      image: ui:$BUILD_NUM
  </code></pre>
  
  <p>Is there a way for me to override the container name (or any service property) that is defined in the base file with a blank value? In my case, I want the ci container name to be randomly generated. I don't want to remove the container name from the base yml file.</p>