Because JS. And somehow we the developers convinced each other that -any- attention to performance whatsoever is "premature optimization".
This has got nothing to do with JS. Plenty of devs (me included) work on frontend and backend JS without containers and everything is instant for us. OP has a special use case or build pipeline necessitating such complexity.
If you change a small bit of css, the whole container is rebuilt? That seems super inefficient to me?
My backends arn't JS, but I still couldn't imagine rebuilding a container everytime I changed a piece of code. Right now I change a file, django picks up the change and reloads. No noticeable delay.