title: How to add class attribute in symfony form
body: <p>I'm looking to add some class attribute in my form using symfony form builder.</p>
  
  <p>I have found how to do this for input and label, but not for the form tag.</p>
  
  <pre><code>$builder-&gt;add('label', TextType::class,
              [
                  'attr' =&gt; ['class' =&gt; 'a'], // for input
                  'label_attr' =&gt; ['class' =&gt; 'b'], // for label
              ]
          );
  </code></pre>
  
  <p>But the render is the following:</p>
  
  <pre><code>&lt;form method="POST"&gt;
        &lt;label for="label" class="b"&gt;Label&lt;/label&gt;
        &lt;input type="text" class="a" name="label"&gt;
         &lt;button type="submit"&gt;Create&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>And i just want </p>
  
  <pre><code>&lt;form method="POST" class="c"&gt;
  </code></pre>
  
  <p>I tried the following:</p>
  
  <pre><code>$builder-&gt;-&gt;setAttribute('class', 'c');
  </code></pre>
  
  <p>But it didn't change anything</p>