title: Docker compose: meaning of {} in volume definition
body: <p>What is the meaning of <code>{}</code> in volume definition?</p>
  
  <p>For example</p>
  
  <pre><code>version: '2'
  
  volumes:
    dataelasticsearch: {}
  
  services:   
    elasticsearch:
      image: docker.elastic.co/elasticsearch/elasticsearch:5.4.3
      volumes:
        - ./dataelasticsearch:/usr/share/elasticsearch/data
  </code></pre>