Docker-compose using host environment variable : <p>I'm trying to do the following in my docker-compose.yml</p>

<p>But I hit this warning?
<code>WARNING: The HOSTNAME variable is not set. Defaulting to a blank string</code></p>

<p><code>environment:
      KAFKA_ADVERTISED_HOST_NAME: ${HOSTNAME}
</code></p>

<p>The HOSTNAME environment variable is obviously set on the host.</p>
