title: How to specify a port number for pm2
body: <p>I'm trying to use pm2 to manage a node.js cluster</p>
  
  <pre><code>pm2 start . -i 3
  </code></pre>
  
  <p>I'm currently running the app on heroku and using a Procfile with the above command, but I cannot figure out how to configure pm2 to use the existing PORT env var. Something like <code>pm2 start . -p $PORT</code></p>
  
  <p>What am I missing?</p>