Add a framework for building Krita under Docker

Now you can have a Krita build in 30 minutes on any Linux OS without
thinkign about the deps (because the deps are prebuilt by KDE).

The docker image uses the same dependencies that are used by KDE CI
for building AppImage builds, therefore the resulting build will look
(and work) almost exactly as the user-delivered AppImage.

CC:kimageshop@kde.org