https://github.com/ospira/docker-django-react-example
In essence, you need two instances of VSCode running connected to two separate Docker container instances. As I understand it, it's one remote container per VSCode window. Thus, I found this to be best, even though it isn't strictly speaking necessary, but it ends up feeling that way because as you said the language server integration (intellisense and extensions) will not work properly if not connected to the right container.
If you load this up in vs code it should prompt you properly given the presence of the files in `.devcontainter` dir. Having two windows in VSCode is kind of annoying at first, but I found it was actually fine, especially on macOS where tabbing to the other VSCode window (as opposed to ungrouped alt+tab on windows) was painless, and also kept me more organized not having backend and frontend code right next to each other.