config_is_read_only does not work

apps/nextcloud/config.php

@@ -3,6 +3,7 @@
 // Manually deployed by yourself
 //
 $CONFIG = array(
+    'config_is_read_only' => true,
     'htaccess.RewriteBase' => '/',
     'memcache.local' => '\\OC\\Memcache\\APCu',
     'apps_paths' => array(
@@ -50,5 +51,6 @@ $CONFIG = array(
     'dbuser' => 'nextcloud',
     'dbpassword' => 'Vb7yHzmE5HIjfU4hf89aXAmEEmxAnMdB',
     'installed' => true,
-    'default_phone_region' => 'DE'
+    'default_phone_region' => 'DE',
+    'updater.release.channel' => 'stable',
 );