[Toaster] Build Appliance replacement

[Barros Pena, Belen]

Now that Hob is gone, the question comes up of what to do with the Build
Appliance

https://www.yoctoproject.org/tools-resources/projects/build-appliance
https://www.yoctoproject.org/documentation/build-appliance-manual

Apparently the plan is to replace it with a Toaster Docker container. It's
also been hinted that such replacement should happen within this release.
So, I guess we need the container, and then we need documentation on how
to set up the container, and then we need to remove the Build Appliance.

I am not sure how we get this started. Opening a Bugzilla issue?

Cheers

Belén