Change gitlab-ci to allow unprivileged gitlab runners

[n-borges]

Description

As is, the gitlab-ci script uses the docker:dind image, which assumes that the runner is privileged. We could change this behaviour, taking advantage of the gitlab container registry to build a test image and pull it later for the test job.

Rationale

Allowing non-privileged runners to run the pipelines would be a good thing securiy-wise.

If you think that might be a good idea, I would be happy to work on a PR for this.