Entity Controllers can get dependencies injected via a createInstance method

Any entity controller (form, access, render, storage etc.) can implement the \Drupal\Core\Entity\EntityControllerInterface
to have their services injected.

Without service injection:

class ExampleEntityViewBuilder extends EntityViewBuilder {

  public function buildContent(array $entities, array $displays, $view_mode, $langcode = NULL) {
    parent::buildContent($entities, $displays, $view_mode, $langcode);

    $site_name = Drupal::config()->get('system.site')->get('name');
  }
}

With proper service injection:

class ExampleEntityViewBuilder extends EntityViewBuilder implements EntityControllerInterface {

  /**
   * Constructs a new ExampleEntityViewBuilder object.
   */
  public function __construct($entity_type, array $entity_info, ConfigFactory $config_factory) {
    parent::__construct($entity_type, $entity_info);

    $this->config = $config_factory->get('system.site');
  }

  /**
   * {@inheritdoc}
   */
  public static function createInstance(ContainerInterface $container, $entity_type, array $entity_info) {
    return new static($entity_type, $entity_info, $container->get('config.factory');
  }

  public function buildContent(array $entities, array $displays, $view_mode, $langcode = NULL) {
    parent::buildContent($entities, $displays, $view_mode, $langcode);

    $site_name = $this->config->get('name');
  }

}