Introduce means for updating latest

This introduces a Dockerfile for updating the Debian base as well
as tlmgr. It is intended to be caching-friendly although it will use
up more space than the previous approach of rebuilding from scratch.

The latest images have to be built from scratch once a new TL is
released, afterwards the updates can take over.

Closes #5.
1 job for !24 with update-images in 1 minute and 22 seconds
latest merge request