nextcloud-docker/15.0-rc/fpm/config/apps.config.php

<?php
$CONFIG = array (
  "apps_paths" => array (
      0 => array (
              "path"     => OC::$SERVERROOT."/apps",
              "url"      => "/apps",
              "writable" => false,
      ),
      1 => array (
              "path"     => OC::$SERVERROOT."/custom_apps",
              "url"      => "/custom_apps",
              "writable" => true,
      ),
  ),
);