code: protected function mergeDefaultConfig(array $config, ContainerBuilder $container)
      {
          $config = array_merge([
              'enable' => true,
              'use_driver' => 'file',
              'private_headers' => ['Authorization', 'Cookie'],
              'etag_hasher' => [],
              'track' => [],
              'drivers' => [],
          ], $config);
  
          $config['etag_hasher'] = array_merge([
              'driver' => 'cache_time_keeper.cache_key_builder.default_etag_hasher',
              'algorithm' => 'sha256',
          ], $config['etag_hasher']);
  
          $config['track'] = array_merge([
              'clear_cache' => true,
              'individually_entity' => false,
          ], $config['track']);
  
          $config['drivers'] = array_merge([
              'multi' => [
                  'fast' => 'shmop',
                  'slow' => 'file',
              ],
              'shmop' => [
                  'salt' => $container->getParameter('cache_time_keeper.driver.shmop.salt'),
              ],
              'file' => [
                  'path' => $container->getParameter('cache_time_keeper.driver.file.path'),
              ],
              'memcache' => [
                  'prefix' => 'cache_time_keeper_',
                  'hosts' => [],
              ],
          ], $config['drivers']);
  
          return $config;
      }
docs: @param array $config
  @param ContainerBuilder $container
  
  @return array