I'm trying to implement the Background Image module, but there seems to be a problem with the dynamic route it registers for generating CSS. I've been debugging it but have run into a wall. It's basically a duplicate of the core Image module's routing setup, so I'm not sure what's wrong, but the controller method never fires - the controller isn't even instantiated. Here's the config file:

route_callbacks:
  - '\Drupal\background_image\Routing\BackgroundImageRoutes::routes'

Here's the routes method:

  public function routes() {
    error_log('Router::routes');
    $routes = [];
    // Generate background image CSS files. If clean URLs are disabled the
    // files will always be served through the menu system. If clean URLs are
    // enabled and the file already exists, PHP will be bypassed.
    $directory_path = $this->streamWrapperManager->getViaScheme('public')->getDirectoryPath();

    $routes['background_image.css'] = new Route(
      '/' . $directory_path . '/background_image/css/{background_image}/{scheme}/{file}',
      [
        '_controller' => '\Drupal\background_image\Controller\BackgroundImageCssController::deliver',
      ],
      [
        '_access' => 'TRUE',
      ]
    );
    error_log(print_r($routes, true));
    return $routes;
  }

Here's the $routes object:

Array
(
    [background_image.css] => Symfony\Component\Routing\Route Object
        (
            [path:Symfony\Component\Routing\Route:private] => /sites/default/files/background_image/css/{background_image}/{scheme}/{file}
            [host:Symfony\Component\Routing\Route:private] => 
            [schemes:Symfony\Component\Routing\Route:private] => Array
                (
                )
            [methods:Symfony\Component\Routing\Route:private] => Array
                (
                )
            [defaults:Symfony\Component\Routing\Route:private] => Array
                (
                    [_controller] => \Drupal\background_image\Controller\BackgroundImageCssController::deliver
                )
            [requirements:Symfony\Component\Routing\Route:private] => Array
                (
                    [_access] => TRUE
                )
            [options:Symfony\Component\Routing\Route:private] => Array
                (
                    [compiler_class] => Symfony\Component\Routing\RouteCompiler
                )
            [compiled:Symfony\Component\Routing\Route:private] => 
            [condition:Symfony\Component\Routing\Route:private] => 
        )
)

If I try to hit a URL such as /sites/default/files/background_image/css/3/public/dq5DbF1hqHxkHXGiHSl9yuXubPox08bOX6nE12gX2h0.css?p2aptq (which the module correctly generates when I've got a background image set up on the site) I get a 404 error and nothing in the error log.

I'm using docker4drupal and have checked the nginx config and am 99% sure that's not the issue (there are no rules for /sites/default/files/ and I do see the 404 in the nginx log.)