Both are now obtained from the repo at pkg.uplex.de, and differ only
in:
- the version, dist and pool parameters of the source repo, as
  expressed in /etc/apt/sources.list, and
- the list of VMODs to install.

We now have one Dockerfile for the two containers, and the four
parameters listed above are passed into the build as build-args,
using values set in the Makefile.

This also has the effect of changing the base image for klarlack
to Debian slim, updated to the currently most recent version.

The Dockerfile is now simpler than the previous version for
klarlack, in that we set the version once in the repo path, rather
than specify "=${VERSION}" for Varnish and each VMOD in the
apt install invocation.