Use pre-built Docker images for CI

Based on an the MR started by Sean 2 years ago. Would be a first step towards #587 (closed)

Docker images are defined in a separate repository https://gitlab.com/ifosim/finesse/docker, which are build and saved to the container registry here https://gitlab.com/ifosim/finesse/docker/container_registry. Pushing a change to this repository will lead to all docker images being replaced. It is also possible to just re-run the latest pipeline, if you just want to get new versions in and not make any changes to the docker files.

Currently the docker images are not tagged, so only copy is ever kept. This reduces the impact on our storage quota, but might need to be changed in the future.

The only dependencies being cached are suitesparse, python build dependencies and latex packages.