title: Environment variables with double asterisks in Dockerfile
body: <p>I have noticed that in some Dockerfile the environment variables are specified using particular expressions, that perform some sort of variable substitution, such as:</p>
  
  <pre><code>ENV PASSWORD **Random**
  ENV NAME **False**
  </code></pre>
  
  <p>I cannot find any reference to those expression in the Docker official documentation.</p>
  
  <p>Where I can find a list of possible expressions that can be used in a Dockerfile and what is their meaning?</p>