Homebox-Actions Container attributes are not real attributes but an array

Looking at the code where we're using the actions type:

$build['homebox_actions_general'] = [
      '#type' => 'actions',
      '#weight' => -100,
      'back_button' => $this->buildBackButton($entity),
      '#attributes' => [
        'class' => [
          'container-inline',
          'homebox-actions',
          'homebox-actions--general',
          'homebox-actions--general-personalize',
        ],
      ],
    ];

and the docs:
https://www.drupal.org/docs/8/theming-drupal-8/using-attributes-in-templ... →
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Template%...

I think we're doing it right. Still I'm not 100% sure.

So this is maybe what you want:

if(is_array($variables['#attributes']['class]) && in_array('homebox-actions', $variables['#attributes']['class])) {
...
}

Still I don't think it's the right way to add a button. Correct would be to use a hook_form_alter() here I guess. But that's a different topic.

Closing this for now until we find other cases where #attributes are not properly handled.