title: Docker compose environment variable is not set
body: <p>In my docker-compose.yml I set the environment variable PORTUNIQUE but it tells me that the env variable isn't set.</p>
  <pre><code>version: &quot;3&quot;
  
  services:
      gateway-service:
        build: ./
        environment: 
          - PORTUNIQUE=8686
        ports:
          - ${PORTUNIQUE}:${PORTUNIQUE}
  </code></pre>
  <p>If I however use a .env file with the following content, it works fine:</p>
  <p><code>PORTUNIQUE=3000</code></p>
  <blockquote>
  <p>WARNING: The PORTUNIQUE variable is not set. Defaulting to a blank
  string.</p>
  </blockquote>