Sweeping !1599 from master to devel: fix: use instructions from https://docs.gitlab.com/ee/ci/docker/using_kaniko.html

I do not know how to test this, but gitlab-registry.cern.ch/ci-tools/docker-image-builder is deprecated (CC7). And we saw this failing in e.g. https://gitlab.cern.ch/lhcb-dirac/LHCbDIRAC/-/jobs/40671380

I just followed "blindly" the instructions in https://docs.gitlab.com/ee/ci/docker/using_kaniko.html