Adding Shadows at the bottom of a container in flutter?

Yes, BoxShadow can solve the problem but instead of manually adding the list of BoxShadow, there is a handy map in flutter call kElevationToShadow which map elevation keys to pre-defined list of BoxShadow. It is also defined based on the material design elevation.

Container(
  height: 60.0,
  decoration: BoxDecoration(
    boxShadow: kElevationToShadow[4],
    color: Theme.of(context).bottomAppBarColor,
  ),
  child: ...
);

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
      ),
    ),

Or you can wrap your Container widget with a Material widget which contains an elevation property to give the shadowy effects.

Container(
          child: Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Material(
                elevation: 15.0,
                child: Container(
                  height: 100,
                  width: 100,
                  color: Colors.blue,
                  child: Center(child: Text("Material",style: TextStyle(color: Colors.white),)),
                ),
              ),
              SizedBox(width: 100,),
              Container(
                height: 100,
                width: 100,
                decoration: BoxDecoration(
                    boxShadow: <BoxShadow>[
                      BoxShadow(
                          color: Colors.black54,
                          blurRadius: 15.0,
                          offset: Offset(0.0, 0.75)
                      )
                    ],
                    color: Colors.blue
                ),
                child: Center(child: Text('Box Shadow',style: TextStyle(color: Colors.white))),
              ),
            ],
          ),
        ),

Image:

Difference between two widgets is shown above. Hope this helps!!!