title: What does this double @ syntax mean in Dockerfile?
body: <p>I'm pretty new to Docker and I'm trying to containerize our web application. Currently we're using Payara Community as our application server.</p>
  <p>The Payara basic Dockerfile is located here:</p>
  <p><a href="https://github.com/payara/Payara/blob/master/appserver/extras/docker-images/basic/src/main/docker/Dockerfile" rel="nofollow noreferrer">https://github.com/payara/Payara/blob/master/appserver/extras/docker-images/basic/src/main/docker/Dockerfile</a></p>
  <p>The first line looks like this:</p>
  <pre><code>FROM @docker.java.image@
  </code></pre>
  <p>I don't fully understand the @@ syntax. I suppose it's a way to insert variables into the Dockerfile but I can't find how to specify them in my own Dockerfile. Would it be possible this way to set a specific JDK of my own choice?</p>