public function build() {
if ($entity_id = $this->configuration['entity']) {
  if (($entity = $this->entityStorage->load($entity_id)) && $entity->access('view')) {
    $render_controller = Drupal::entityTypeManager()->getViewBuilder($entity->getEntityTypeId());
    $view_mode = isset($this->configuration['view_mode']) ? $this->configuration['view_mode'] : 'default';

    if (isset($this->configuration['card_colour'])) {
      $entity->card_colour = $this->configuration['card_colour'];
    }
    if (isset($this->configuration['card_layout'])) {
      $entity->card_layout = $this->configuration['card_layout'];
    }

    return $render_controller->view($entity, $view_mode);
  }
}

return [];

{%
    set classes = [
        'node-card',
        'card-' ~ node.card_layout|clean_class,
        node.card_colour|clean_class,
        'clearfix'
    ]
%}
<article{{ attributes.addClass(classes) }}>
    <div{{ content_attributes.addClass('bg-img', 'clearfix') }} style="background-image: url()">
        <div class="summary">
            {{ content.body }}
        </div>
    </div>
</article>

$entity = clone $entity;