title: Re-using existing volume with docker compose
body: <p>I have setup two standalone docker containers, one runs a webserver another one runs a mysql for it.
  Right now I was attempting to have it working with docker-compose. All is nice and it runs well, but I was wondering how could I re-use existing volumes from the existing standalone containers that I have previously created (since I want to retain the data from them).</p>
  
  <p>I saw people suggesting to use <code>external: true</code> command for this, but could not get the right syntax so far.</p>
  
  <p>Is <code>external: true</code> the correct way approach for this, or should I approach this differently?
  Or can I just specify the path to the volume within docker-compose.yml and make it use the old existing volume?</p>