KnpMenuBuilder dropdownlist in menu (Bootstrap) - php

Question

In my current web application I am making use of KnpMenuBuilder and Bootstrap to make a menu. I want to add an unordered list to a list item element, so that it appears as a dropdown menu, just like the example(s) given in the Bootstrap documentation. For this I should also add a dropdown-menu class to this <ul></ul> tag. It should look something like this:
<ul>
<li>
...
</li>
<li><!-- or this should be <li class="dropdown"> not entirely sure
if this happens with javascript or something yet -->
<a href="#" class="dropdown-toggle">
Dropdown
</a>
<ul class="dropdown-menu">
<li>Dropdown menu item 1</li>
<li>Dropdown menu item 2</li>
</ul>
</li>
</ul>
I know you can add such element in the MenuBuilder class:
public function mainMenu(FactoryInterface $factory, array $options)
{
$menu = $factory->createItem('root', array(
'childrenAttributes' => array(
'class' => 'nav navbar-nav',
),
));
$menu->addChild('Home', array('route' => 'home'));
$menu->addChild('Movies', array('route' => 'movies'))
->setAttribute('class', 'dropdown');
$menu['Movies']->addChild('Add a movie', array('route' => 'movie_create_form'));
//This renders <li>movies<ul><li>Add a movie</li></ul></li>
//I want to add the drop-down class to the <ul></ul> tag
return $menu;
}
My current result:
<!-- Ofcourse <ul> tag should be around this aswell, but my question is not about the beauty of this code -->
<li>...</li>
<li dropdown="dropdown" class="active last">
<a href="#">
Dropdown
</a>
<ul class="menu_level_1">
<li class="first last">
<a href="#">
Dropdown menu item 1
</a>
</li>
</ul>
</li>
Or: http://pastebin.com/XHyYeWer
(original: http://pastebin.com/gnK2G0uz)
How should I change my (builder class) code?
Thanks in advance.