flatdeb-builder: Add new builder

This allows to use flatdeb to build Mildenhall runtime and SDK, using kvm or UML runners.

ARG RELEASE should be declared a second time after FROM to be accessible by RUN commands, cf. https://ryandaniels.ca/blog/docker-dockerfile-arg-from-arg-trouble/

Signed-off-by: Frédéric Danis frederic.danis@collabora.com