Suggestion: provide an "official" Sming build container

[pljakobs]

I recently finally built my own CI for my project and one thing that I had to solve for was not to create a fresh Sming build environment for every single build run.

I decided to create a container with Sming the necessary toolchains fully installed, submodules intiialized and at least the core modules built for every SMING_SOC. This way, when I do a matrix build, I only have to pull that container to have everything in place.

It would be great to have that container built right from the Sminghub/Sming repo, ideally for the relevant branches (releases, develop, master), have them uploaded to a container registry so that other CIs could just pull them as an up-to-date build environment.

If you're happy to do that, I'd be happy to help out with it.