title: docker-compose.yml content - How can i avoid "must be a mapping not a string" error message?
body: <p>Problem: the below content returns "service 'image' must be a mapping not a string." 
  I tried using YAML Parser(<a href="http://yaml-online-parser.appspot.com/" rel="nofollow noreferrer">http://yaml-online-parser.appspot.com/</a>), but it returned no error. </p>
  
  <pre><code>version: 
       "2.0"
  
  services:
  
   blog:
  
   image: 
    abc/defg
   environment:
    APPLICATION_SECRET:
     82xxxxxxx
  
    ports: -"9000:9000"
  </code></pre>
  
  <p>working version:</p>
  
  <pre><code>version: "2.1"
  services:
   blog:
    image:  abc/defg
    environment:
     APPLICATION_SECRET:
      82xxx
    ports: 
     - "9000:9000"
  networks:
    default:
      external:
        name: nat
  </code></pre>