title: displaying help messages while docker build
body: <p>I am building a Dockerfile and I would like to display some help messages while building it.</p>
  
  <p>I also tried <code>RUN echo "installing this"</code> but as expected, it doesn't work.</p>
  
  <p>So, How do I display help messages and if possible while running the <code>docker build</code> command in quiet mode.</p>