code: protected function mergeBackwardCompatibilityConfig(array $config, ContainerBuilder $container)
      {
          if ($container->hasParameter('cache_time_keeper.driver')) {
              $config['use_driver'] = $container->getParameter('cache_time_keeper.driver');
          }
  
          foreach (['fast', 'slow'] as $item) {
              if (empty($config['drivers']['multi'][$item]) &&
                  $container->hasParameter('cache_time_keeper.driver.multi.'.$item)
              ) {
                  $config['drivers']['multi'][$item] = $container->getParameter('cache_time_keeper.driver.multi.'.$item);
              }
          }
  
          return $config;
      }
docs: @param array $config
  @param ContainerBuilder $container
  
  @return array