EntityTypeInfo.php

<?php

namespace Drupal\my_dummy_module;

use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
use Drupal\Core\Session\AccountInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Manipulates entity type information.
 *
 * This class contains primarily bridged hooks for compile-time or
 * cache-clear-time hooks. Runtime hooks should be placed in EntityOperations.
 */
class EntityTypeInfo implements ContainerInjectionInterface {

  /**
   * Namespace of 'DUMMY FORM' NodeForm overrides.
   */
  const MY_DUMMY_FORM = 'Drupal\my_dummy_module\Form\MyDummyForm';

  /**
   * The current user.
   *
   * @var \Drupal\Core\Session\AccountInterface
   */
  protected $currentUser;

  /**
   * EntityTypeInfo constructor.
   *
   * @param \Drupal\Core\Session\AccountInterface $current_user
   *   Current user.
   */
  public function __construct(AccountInterface $current_user) {
    $this->currentUser = $current_user;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('current_user')
    );
  }

  /**
   * Add new properties onto entity types.
   *
   * This is an alter hook bridge.
   *
   * @param \Drupal\Core\Entity\EntityTypeInterface[] $entity_types
   *   The master entity type list to alter.
   *
   * @see hook_entity_type_alter()
   */
  public function entityTypeAlter(array &$entity_types) {
    if ($entity_types['node']->getFormClass('dummy_mode')) {
      $entity_types['node']->setFormClass('dummy_mode', static::MY_DUMMY_FORM);
    }
  }

}

my_dummy_module.info.yml

name: My dummy module
type: module
description: Module presentation...
core: 8.x
package: CUSTOM
dependencies:
  - drupal:form_mode_manager

my_dummy_module.module

<?php

/**
 * @file
 * The my_dummy_module module.
 */

use \Drupal\my_dummy_module\EntityTypeInfo;

/**
 * Implements hook_entity_type_alter().
 */
function my_dummy_module_entity_type_alter(array &$entity_types) {
  return \Drupal::service('class_resolver')
    ->getInstanceFromDefinition(EntityTypeInfo::class)
    ->entityTypeAlter($entity_types);
}

MyDummyForm.php

<?php

namespace Drupal\my_dummy_module\Form;

use Drupal\Component\Datetime\TimeInterface;
use Drupal\Core\Entity\EntityManagerInterface;
use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\node\NodeForm;
use Drupal\user\PrivateTempStoreFactory;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Form handler for the node content "dummy_mode" edit/add forms.
 */
class MyDummyForm extends NodeForm {

  /**
   * Constructs a MyDummyForm object.
   *
   * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
   *   The entity manager.
   * @param \Drupal\user\PrivateTempStoreFactory $temp_store_factory
   *   The factory for the temp store object.
   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
   *   The entity type bundle service.
   * @param \Drupal\Component\Datetime\TimeInterface $time
   *   The time service.
   */
  public function __construct(EntityManagerInterface $entity_manager, PrivateTempStoreFactory $temp_store_factory, EntityTypeBundleInfoInterface $entity_type_bundle_info = NULL, TimeInterface $time = NULL, AccountInterface $current_user) {
    parent::__construct($entity_manager, $temp_store_factory, $entity_type_bundle_info, $time, $current_user);
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('entity.manager'),
      $container->get('user.private_tempstore'),
      $container->get('entity_type.bundle.info'),
      $container->get('datetime.time'),
      $container->get('current_user')
    );
  }

  /**
   * {@inheritdoc}
   *
   */
  public function form(array $form, FormStateInterface $form_state) {
    $form = parent::form($form, $form_state);
    $form['my_dummy_wrapper'] = [
      '#type' => 'details',
      '#title' => t('Wrapper custom'),
      '#open' => TRUE,
      '#attributes' => [
        'class' => ['node-form-dummy'],
      ],
      '#attached' => [
        'library' => ['node/drupal.node'],
      ],
      '#group' => 'advanced',
    ];

    $form['field_image']['#group'] = 'my_dummy_wrapper';
    return $form;
  }

}