Add devcontainer manifest for our CI image!
Devcontainer [1] is a popular container schema that makes IDEs and tooling aware about the container image you want to use and develop against.
The most popular implementation of devcontainer are Visual Studio and VSCode and this allows us to have a out of the box working pre-defined and working build environment that is very close to the environment CI runs against.
This also allows VSCode on Windows and Mac to setup a Linux docker container (vm) to develop against if so desired.