title: Heroku run Docker image with port parameter
body: <p>When I push an existing Docker image to Heroku, Heroku provides a <code>$PORT</code> environment variable. How can I pass this property to the Heroku run instance?</p>
  
  <p>On <code>localhost</code> this would work:</p>
  
  <pre><code>docker pull swaggerapi/swagger-ui
  docker run -p 80:8080 swaggerapi/swagger-ui
  </code></pre>
  
  <p>On Heroku I should do:</p>
  
  <pre><code>docker run -p $PORT:8080 swaggerapi/swagger-ui
  </code></pre>
  
  <p>Is something like this possible?</p>