this needs to be:

````xml
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:cmf-menu="http://cmf.symfony.com/schema/dic/menu">
    <cmf-menu:config xmlns="http://cmf.symfony.com/schema/dic/menu">
        <!-- ... -->
    </cmf-menu:config>
</container>
````