title: Docker driver_opts.type=none?
body: <p>What does type=none do? e.g.</p>
  
  <pre><code>volumes:
    transmission_config:
      driver: "local"
      driver_opts:
        type: "none"
  </code></pre>
  
  <p><a href="https://github.com/search?l=yaml&amp;p=6&amp;q=%22driver_opts%22&amp;type=Code" rel="noreferrer">Lots of people</a> are using it but I can't find where any of this is documented.</p>
  
  <p>Where are all the driver options documented?</p>