FormType.php

class TricksType extends AbstractType
{
    private $entitymanager;

    public function __construct(EntityManagerInterface $entityManager)
    {
        $this->entitymanager = $entityManager;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        // On affiche les champs du builder
        $builder
            ->add('nom', TextType::class)
            ->add('description', TextareaType::class)
            ->add('groupe', GroupeAddType::class)


            ->add('image', CollectionType::class, array(
                'entry_type' => ImageAddType::class,
                'entry_options' => array('data_class' => Image::class, 'label' => false),
                'allow_add' => true,
                'allow_delete' => true,
                'by_reference' => false,

                /*'mapped' => false,*/
    ));

    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => Tricks::class
        ));
    }
}

class ImageAddType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('alt', TextType::class, array('label' => 'Mot clefs(alt)'))
            ->add('url', FileType::class, array('label' => 'Ajouter une image:',
                ));

    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => Image::class,
        ));
    }
}