The old Dockerfile called `ServerMain` directly using a small selection
of environment variables with outdated names. It was also outdated in
other respects.

The new Dockerfile installs the `qlever` script, so that it can be
called from inside the container.

Remaining questions / TODOs, feedback welcome:

1. Right now, the script is installed as part of the docker build via
   `pipx install qlever`. Is this the right way to do it?
   Alternatives would be to clone the GitHub repo and `pipx install -e
   .` from there, or include the the GitHub repo as a submodule of this
   repository.

2. How do we handle the QLever UI. We could just call `qlever ui` from
   inside the container. But that would pull the Docker image for the
   Qlever UI and run a Docker container inside of a Docker container.
   It's possible, but not the right way to do it. If both are needed,
   the container for the QLever engine and the contaner for the QLever
   UI should run side by side.

3. The`qlever setup-config` command should have options that overwrite
   the variables in the produced Qleverfile. In particular, there should
   be an option for setting `SYSTEM = native`. Otherwise it has to be
   stated explictly for each command, where that is relevant (in
   particular: `qlever index`, `qlever start`, `qlever
   example-queries`).