buildroot

support: add Dockerfile for CI

For Gitlab-CI, we want to avoid re-generating the minimal install to
be able to run tests all the time. So let's create a docker image that
we can post on Docker Hub and then pull.

For the time being, this is just what we need for running our CI. Later
we can produce something that is also useful for users.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>