title: Flutter: bottom center widget in stack
body: <p>I wanna achieve like this pic. to simplify I created a stack that contains two Containers. i want to bottom center the small container using Align widget but its not working! it is always remaining at the top </p>
  
  <pre><code>Stack(
                overflow: Overflow.visible,
                children: &lt;Widget&gt;[
                  Container(
                    width: MediaQuery.of(context).size.width,
                    height: 170,
                    decoration: BoxDecoration(
                        color: Colors.white,
                        borderRadius: BorderRadius.only(
                            bottomLeft: Radius.circular(0),
                            bottomRight: Radius.circular(0))),
                    // child: Image.network(tutorImage),
                  ),
                  Align(
                    alignment: Alignment.bottomCenter,
                    child: Container(
                      width: 60,
                      height: 60,
                      color: Colors.black,
                    ),
                  )
                ],
              ),
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/jvc4Q.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/jvc4Q.jpg" alt="enter image description here"></a></p>