title: Dockerfile ARG substitution in a string in RUN command
body: <p>In my Dockerfile I want to substitute a variable in a string.</p>
  
  <pre><code>ARG w=world
  RUN echo 'Hello $w'
  </code></pre>
  
  <p>I want the output to be <code>Hello world</code> but the actual output is <code>Hello $w</code></p>