docs: Add a small update noting either a subdomain or arbitrary domain.

docs/content/users/extend/custom-compose-files.md

@@ -67,7 +67,7 @@ When defining additional services for your project, we recommend following these
 
     * To expose a web interface to be accessible over HTTP, define the following environment variables in the `environment` section for docker-compose:
 
-        * `VIRTUAL_HOST=$DDEV_HOSTNAME` You can also specify an arbitrary hostname like `VIRTUAL_HOST=extra.ddev.site`.
+        * `VIRTUAL_HOST=$DDEV_HOSTNAME` You can set a subdomain with `VIRTUAL_HOST=mysubdomain.$DDEV_HOSTNAME`. You can also specify an arbitrary hostname like `VIRTUAL_HOST=extra.ddev.site`.
         * `HTTP_EXPOSE=portNum` The `hostPort:containerPort` convention may be used here to expose a container’s port to a different external port. To expose multiple ports for a single container, define the ports as comma-separated values.
         * `HTTPS_EXPOSE=<exposedPortNumber>:portNum` This will expose an HTTPS interface on `<exposedPortNumber>` to the host (and to the `web` container) as `https://<project>.ddev.site:exposedPortNumber`. To expose multiple ports for a single container, use comma-separated definitions, as in `HTTPS_EXPOSE=9998:80,9999:81`, which would expose HTTP port 80 from the container as `https://<project>.ddev.site:9998` and HTTP port 81 from the container as `https://<project>.ddev.site:9999`.