title: Dockerfile Overriding ENV variable
body: <p>I have a Dockerfile and I'd like to make the API configurable with a default value.</p>
  
  <blockquote>
    <p>FROM socialengine/nginx-spa</p>
    
    <p>ENV API_URL localhost:6007</p>
  </blockquote>
  
  <p>So when I run this image I'd to be able to override the localhost:6007 with something like below:</p>
  
  <blockquote>
    <p>docker run -e API_URL=production.com:6007 ui</p>
  </blockquote>
  
  <p>This doesn't work and I can't find a clear explanation of how to do this. </p>
  
  <p>Any advice?</p>