Base URL option in generate form doesn't work when running Drupal in a sub-directory

I've got a quick hack working with an outbound path processor service:

class SiteRootOutboundPathProcessor implements OutboundPathProcessorInterface {

  /**
   * {@inheritdoc}
   */
  public function processOutbound($path, &$options = [], Request $request = NULL, BubbleableMetadata $bubbleable_metadata = NULL) {
    if (!$request) {
      // WTF? Why is $request sometimes NULL?
      return $path;
    }

    if (!$request->attributes->has(StaticGeneratorInterface::REQUEST_KEY)) {
      return $path;
    }


    // Hardcoded for now - should get this value from the base URL option in the form.
    $options['base_url'] = 'http://localhost:8888/tome-blog/html/tome-blog/web';

    return $path;
  }

}