Use gitlab builds for master images

[coudot]

We can now use latest builds from OW2 gitlab instead of building packages in Dockerfile : https://gitlab.ow2.org/lemonldap-ng/lemonldap-ng/-/jobs/artifacts/master/download?job=build-debian

[guimard]

1 problem: link is available only if at least 1 commit has been pushed in the last 7 days