how to add a form to a build array I am creating a custom module that has the following as the build:

$build = array(

          '#type' =&gt; 'markup',
          '#markup' =&gt; $html,
          '#attached' =&gt; array(

             'library' =&gt; array('pup_viewfiles/sortfiles_libraries'),
            ),
        );

    return $build;


I have a Form that I get with the following line : 

$form=\Drupal::formBuilder()-&gt;getForm(pup_viewfiles\pager),


The question is how can I incorporate this form into the build ?
getForm() returns a render array, and a render array can have other render arrays as children, so:

$build = [
    'form' =&gt; $form,
    ...
];