[Docker] Provide AlekSIS base image

Currently, we are building all docker images from the AlekSIS distribution repository.

We should instead provide a base image from the AlekSIS core, to ease building of reference images in single apps.

added part::packaging type::feature labels

created merge request !511 (merged) to address this issue

mentioned in merge request !511 (merged)

closed with merge request !511 (merged)

mentioned in commit 5b41df20