Flutter: Trying to bottom-center an item in a Column, but it keeps left-aligning

I'm trying to bottom-center a widget at the bottom of a Column, but it keeps aligning to the left.

return new Column(
  new Stack(
    new Positioned(
      bottom: 0.0, 
      new Center(
        new Container(),
      ),
    ),
  ),
);

The existence of the Positioned forces the Container to the left, instead of centering. Removing the Positioned, however, puts the Container in the middle-center.