env variables used:

DOCKER_LOGIN_USERNAME
DOCKER_LOGIN_PASSWORD
set in /etc/gitlab/gitlabregistry.txt managed by Puppet

This is needed because gitlabregistry does not implement search.
therefore, one needs to list images through gitlab curl API using JSON
Web Token.
This is needed for tests outside of CI, were one must deduce ctageneric
docker image tag from commit ID, the format is <pipeline ID>git<commit
ID> and while we have access to commit ID, we cannot guess pipeline ID.
The idea is then to run the tests with the highest pipeline ID and the
commit ID of HEAD.