Add a dockerfile and a script building "slim" AuTA images

Description

Adds a slim.Dockerfile and a script for building images containing only the bare necessities for running the components of AuTA.

It generates two separate images: one containing the core, the other the worker. The interface still matches the one in the readme, although the path to the worker image has changed (and the core image is entirely new of course).

This MR has been marked as no-code, because it contains no testable Java code, just a shell script and a Dockerfile. Still, I can assure you that both have been tested: both images are in service on Zaphod right now!

Changes

None.

Additions

• A slim.Dockerfile that applies to both the core and the worker (through some weird ARG magic)
• A build-slim-image file that can build either image, or both. Read how to use it here: Deploying to Docker

Test and Review

To be filled in by the reviewers

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date