title: How do I add :cached or :delegated into a docker-compose.yml volumes list?
body: <p>Title says it all. I've got a few volumes set up as readonly (<code>:ro</code>) but want to test <code>:cached</code> and <code>:delegated</code> for helping with file i/o performance, but couldn't figure out how to set this up in a compose file.</p>
  
  <p>Oh, I already tested:</p>
  
  <p><code>volumes:
   - external:internal:cached
  </code></p>