Checkboxes in a simple form are not displayed
You code works for me. I used the exact following code and got checkboxes in my form using Drupal 8.3.4:

$form['container'] = [
  'categories' =&gt; [
    '#title' =&gt; t('Select a Category'),
    '#type' =&gt; 'checkboxes',
    '#options' =&gt; [
      1 =&gt; t('Option 1'),
      2 =&gt; t('Option 2'),
      3 =&gt; t('Option 3'),
    ],
  ],
];

return $form;