title: Docker-Copmose.yml environment variable default value computation
body: <p>I have a php docker image that I'm initiating from the docker compose with some additional containers.</p>
  
  <p>I would like to enable the php remote debugging on the container and I don't want to have any hard coded configuration values.</p>
  
  <p>the hard coded values works fine</p>
  
  <pre><code>services: 
      phpSite: 
          environment:
              XDEBUG_CONFIG: remote_host=${MY_IP:-1.1.1.113}
  </code></pre>
  
  <p>I know that I can get my local ip from the ipconfig on my host name but I have failed to solve it dynamically</p>