how to put data form database into checkbox symfony 3

I started learn symfony 3. For the first project i chose a simple totodlist.

So now i have possibility to create and save the user in my database. Next I can create a task for them.

I want to create a checkbox where can i choose a users to perform a task.

So i need put data from my user database to checkbox form ($temp_users varbiable). I don't know how to do it.

Can anybody show me how to do it.

below is my code:

public function createAction(Request $request)
{

    $todo = new Todo;

    $users = $this->getDoctrine()
    ->getRepository('AppBundle:User')
    ->findAll();

    $temp_users = array();
    foreach($users as $user) {
    $temp_users[$user->getUsername()] = $user->getId();

}

 $form = $this->createFormBuilder($todo)
     ->add('name', TextType::class, array('attr' => array('class' => 'form-     control', 'style' => 'margin-bottom:15px')))


    ->add('wykona', CheckboxType::class, array('label' => $temp_users, 'required' => false,))