Adding Shadows at the bottom of a container in flutter?

You can reuse the first container that you have in your Stack, the container has a property called decoration and it accept a widget of kind BoxDecoration, as you can see in this link: BoxDecoration
Inside this widget you can use the boxShadow property to give to your container a custom shadow, try the next code:

new Container(
      height: margin_100dp,
      decoration: BoxDecoration(
          boxShadow: <BoxShadow>[
            BoxShadow(
                color: Colors.black54,
                blurRadius: 15.0,
                offset: Offset(0.0, 0.75)
            )
          ],
        color: colorPrimary
      ),
    ),