Invalid upload directory for EasyAdmin ImageField once deployed on Heroku

Question

I work on a Symfony 6.0.9 website with EasyAdmin to handle the administration panel. I've got an entity User and an entity MediaObject that can be an image for example. The User has a property $profilePicture which is a MediaObject. In EasyAdmin, I've got a UserCrudController like that :

class UserCrudController extends AbstractCrudController
{
    public static function getEntityFqcn(): string
    {
        return User::class;
    }

    public function configureCrud(Crud $crud): Crud
    {
        ...
    }

    public function configureFields(string $pageName): iterable
    {
        return [
            ...

            FormField::addPanel('Pictures')->renderCollapsed(),
            ImageField::new('profilePicture.filePath')
                ->setBasePath('/media')
                ->setUploadDir('public/media')
                ->setUploadedFileNamePattern('profilePicture.[extension]')
                ->setLabel('Profile picture')
                ->setColumns(4),
            ...
        ];
    }
}

In my app/config/services.yaml file, I've set :

parameters:
    app.path.media_object: /media

In my app/config/packages/vich_uploader.yaml file, I've set :

vich_uploader:
    db_driver: orm

    mappings:
        media_object:
            uri_prefix: '%app.path.media_object%'
            upload_destination: '%kernel.project_dir%/public%app.path.media_object%'

When I'm in my localhost (so in dev env), everything works just fine. When I build and deploy on Heroku, I works fine. But when I try to edit an user, I've got this error message in Heroku's logs :

[critical] Uncaught PHP Exception Symfony\Component\OptionsResolver\Exception\InvalidArgumentException: "An error has occurred resolving the options of the form "EasyCorp\Bundle\EasyAdminBundle\Form\Type\FileUploadType": Invalid upload directory "/app/public/media/" it does not exist or is not writable." at /app/vendor/symfony/form/ResolvedFormType.php line 93

I don't understand what I've done wrong. Thanks for helping me ! ;)