container_service_providers can't be extended

See following code in globals.api.php

/**
 * Allows defining of site-specific service providers for the Drupal kernel.
 *
 * To define a site-specific service provider class, use code like this:
 * @code
 * $GLOBALS['conf']['container_service_providers']['MyClassName'] = 'Drupal\My\Namespace\MyClassName';
 * @endcode
 *
 * @see \Drupal\Core\DrupalKernel::$serviceProviderClasses
 */
global $conf;

Patch attached fixes the problem, but is not a correct solution.