WIP: Faster CI
The main goal of this merge request is to speed up the gitlab CI by using pre-built docker images instead of having to download and install the software each time.
I also added some other linters, with apppropriate configuration:
- Ansible linter
- Docker linter for the nk20 Dockerfile
- Docker linter for the CI Dockerfiles
It made sure to separate these commits to make it optional and easily revertible.
- Average runtime before this MR: ~12 min
- Average runtime after this MR (without newly added linters): ~8 min
- Average runtime after this MR (with newly added linters): ~7m30
Note: this is a still a prove of concept and uses dockerhub, while it would be better to use the local gitlab registery. The gitlab docker registery is not yet available.