title: Reference variables from a base DockerFile in child dockerfiles?
body: <p>I am currently working on splitting a huge DockerFile into separate DockerFiles but some variables that are defined in the huge DockerFile needs to be used in the separate DockerFiles. Does Docker have a concept of importing common variables across a set of DockerFiles or some other inheritance mechanism?</p>
  
  <p>I would like to avoid having to maintain duplicate variables across multiple dockerfiles.</p>
  
  <p>As an example:</p>
  
  <pre><code>ARG basic_user="user"
  USER ${basic_user}
  </code></pre>