Add css class to the container of the action button from a Form

Question

I have a contact form in a Block, using the contact_block module, now I want to add some bootstrap classes to the div container of the actions buttons (send message).

At the end of the form it I got this wrapper:

<div data-drupal-selector="edit-actions" class="form-actions form-group js-form-wrapper form-wrapper" id="edit-actions">
<button class="btn btn-primary btn-xl text-uppercase button button--primary js-form-submit form-submit btn-primary btn" data-drupal-selector="edit-submit" type="submit" id="edit-submit" name="op" value="Send message">Send message</button>
</div>

I want to affect the div id="edit-actions".

According to twig debug, I should do a hook of the container, but I don't know how to specify that I want to affect that div.

Inside the themes/bootstrap/templates/system/container.html.twig is the structure of the containers, so I can add some classes there, but I don't know how to specify which container I want to affect. I had tried to change the name to container--contact_message_feedback_form.html.twig, but it doesnt work.

{%
  set classes = [
    not attributes.hasClass('toolbar-menu-administration') ? 'form-group',
    has_parent ? 'js-form-wrapper',
    has_parent ? 'form-wrapper',
  ]
%}
<div{{ attributes.addClass(classes) }}>{{ children }}</div>

Extra info: Drupal 8.4

Subtheme bootstrap

I have already a hook_form_alter() to remove the preview button.

function twig_block_form_alter(&$form, $form_state, $form_id) {

  if (strpos($form_id, 'contact_message_feedback_form') !== FALSE) {
    $form['actions']['preview']['#access'] = FALSE;
    $form['actions']['submit']['#attributes']['class'][] = 'btn btn-primary btn-xl text-uppercase';
    // Adding prefix and suffix to the form.
    $form['elements']['edit-actions']['#prefix'] = '<div class="col-md-12 text-center">';
    $form['elements']['edit-actions']['#suffix'] = '</div>';


  }
}

Answer 1

Thanks to kint for devel options:

There you go:

/**
 * Implements hook_form_alter().
 */
function twig_block_form_alter(&$form, $form_state, $form_id) {

  if (strpos($form_id, 'contact_message_feedback_form') !== FALSE) {
    $form['actions']['preview']['#access'] = FALSE;
    $form['actions']['submit']['#attributes']['class'][] = 'btn btn-primary btn-xl text-uppercase';
    // kint($form['actions']['submit']);
    $form['actions']['submit']['#prefix'] = '<div class="col-md-12 text-center">';
    $form['actions']['submit']['#suffix'] = '</div>';
  }
}