Switch to v2020 Docker as v2021dev2 images need special UML options

The Jenkins workers haven't gained yet the special options to be able to run our new UML-based images, /scratch tmpfs and CAP_PTRACE, so the builds currently fail there as the KVM backend is disabled.

Revert the 6319aa99 "Use images from the Gitlab Docker registry" commit and swith back to the v2020 images while the Jenkins workers get updates and we've not moved this pipeline to GitLab CI yet.