title: Docker-compose set subnet as variable results to "should use the CIDR format"
body: <p>I am setting the subnet and gateway in my docker-compose file as variables. It looks like this:</p>
  <pre><code>networks:
      btgnetwork:
          driver: bridge
          ipam:
              driver: default
              config:
                  - subnet: ${STAGE_NETWORK_IP}.0/24
                    gateway: ${STAGE_NETWORK_IP}.1
  </code></pre>
  <p>The variable is set in another file called &quot;compose-vars.env&quot; with <code>STAGE_NETWORK_IP=192.168.2</code></p>
  <p>But this does not work. The Error is:</p>
  <pre><code>ERROR: The Compose file './docker-compose.yml' is invalid because:
  networks.btgnetwork.ipam.config.subnet is invalid: should use the CIDR format
  </code></pre>
  <p>I cloud not find any useful tips for that. Thank you :)</p>