Use another box-shadow?
You can have more than one.
Frankly though you’re after “full width browser bars” and Chris has an article on that:
https://css-tricks.com/full-browser-width-bars/
To my mind though, it’s a lot of hassle that can simply be fixed by closing the “container”…have a new 100% wide div..and opening a new container after that.
Then you can add a centered inner container inside the full width bar.
“container” is just a class…not an absolute rule that everything must be inside one single container.