title: Double-Double Curly Braces in docker-compose.yml for kafka
body: <p>I'm looking at wurstmeister/kafka-docker, and found this <a href="https://github.com/wurstmeister/kafka-docker/pull/377/commits/1966b1027e7f6d296b7a1e2b94d61c35316888b4" rel="nofollow noreferrer">pull</a> suggesting using the IP address.  Basically, it uses the docker info formatting: </p>
  
  <pre><code>HOSTNAME_COMMAND: "docker info -f '{{`{{.Swarm.NodeAddr}}`}}'"
  </code></pre>
  
  <p>I get the idea that it uses the formatting to get the address, but I don't understand what's going on with the extra {{...}}  and the backticks (which would normally be an eval).  docker-compose allows for ${variables}, but there's no dollar sign ($) here, so no need to escape.  Why isn't this just: </p>
  
  <pre><code>HOSTNAME_COMMAND: "docker info -f '{{.Swarm.NodeAddr}}'"
  </code></pre>
  
  <p>I've seen this elsewhere, so I assume there's a reason.</p>