title: change angular env variables at runtime
body: <p>I would like to be able to change the url on which my angular will fetch ressources from the server at build time. I would to pass a parameter during the build to change the url value
  The reason behind is for deployment in different docker containers.</p>
  
  <p>environment.docker.ts</p>
  
  <pre class="lang-js prettyprint-override"><code>export const environment = {
    ...
    url: "localhost:8080"
  };
  </code></pre>
  
  <p>I would like to have something like : <code>ng build --env docker --url:"localhost:9090"</code> which will update the default value during build time.</p>