Deal with the lack of support for CentOS7

From 30.06.2024, the support for CentOS7 has been dropped. The Linux support images are now mirrors of the CentOS7 upstream project, and changes to the latter will be propagated accordingly. We must adapt the Dockerfile so we avoid updating packages that are no longer available (see e.g. https://gitlab.cern.ch/lhcb-docker/analysis-ci/-/jobs/41980373), while still keeping a lxplus-legacy-like environment.

The temporary solution is to use a personal repository in Docker Hub to host the images, which should eventually be moved to a LHCb/CERN-managed repository (see lhcb-dpa/wp6-analysis-preservation-and-open-data/wp6-apandod#53). This repository will simply pull the images from there and push them to the container registry of this repository. In the future, changes to the GitLab container registry might allow having a more protected storage which prevents accidental removal of the images (see documentation).