title: Adding default external network in docker-compose
body: <p>I am trying to learn docker and understanding docker-compose</p>
  
  <p>As I was trying out the external network section:</p>
  
  <pre><code> networks:
    default:
      external:
        name: my-pre-existing-network
  </code></pre>
  
  <p>I understand that 
  'my-pre-existing-network' needs to be created.</p>
  
  <p>Is it possible to create a new default external network from within the compose file itself?</p>
  
  <p>This is more from a learning/understanding perspective And also alternative to the docker network create command. Thanks.</p>