code: protected function initializeContainer()
      {
          $this->registerDefaultExtensions();
          $initializer = $this->getContainerInitializer();
          $this->container = $initializer->initializeContainer($this, $this->extensions, $this->compilerPasses);
          $this->container->set('app', $this);
  
          return $this->container;
      }
docs: Initializes a new container.
  
  @return ContainerInterface