title: docker-compose.yml, network must be a mapping, not an array
body: <p>Not sure how the yaml indentation works , getting an error, while define an external network for the app to communicate
  ./docker-compose.yml', network must be a mapping, not an array</p>
  <pre><code>    version : '3'
  
      services:
         zmq_sub:
           image: zmq_sub
         zmq_pub:
           image: zmq_pub
           depends_on:
             - zmq_sub 
  
      networks:
         - zmq_network:
           external: true
  </code></pre>