provisioning/dockerfiles/services/nextcloud/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,
        ),
    ),
);