$form = $this->createFormBuilder($article)
            ->add('title', TextType::class, array('attr' => array('class' => 'form-control')))
            ->add('body', TextareaType::class, array(
                'required' => false,
                'attr' => array('class' => 'form-control')
            ))
            ->add('save', SubmitType::class, array(
                'label' => 'Create',
                'attr' => array('class' => 'btn btn-primary mt-3')
            ))->getForm();

<div class="container">
        {{ form_start(form) }}
        {{ form_widget(form) }}
        {{ form_end(form) }}
    </div>