title: Docker Compose: volumes without colon (:)
body: <p>I have a <code>docker-compose.yml</code> file with the following:</p>
  
  <pre><code>volumes:
    - .:/usr/app/
    - /usr/app/node_modules
  </code></pre>
  
  <p>First option maps current host directory to <code>/usr/app</code>, but what does the second option do?</p>