Builds a simple form to test the #group property on #type 'container'.
function form_test_group_container() {
$form['container'] = array(
'#type' => 'container',
);
$form['meta'] = array(
'#type' => 'details',
'#title' => 'Group element',
'#group' => 'container',
);
$form['meta']['element'] = array(
'#type' => 'textfield',
'#title' => 'Nest in details element',
);
return $form;
}