Gitlab CI Runner version mismatch between Large Runner and Whitewhale

Hi!

While I was investigating a bug on Large Richard (jvoisin/mat2#67 (closed) - the job works on Whitewhale and not on Large Richard), I saw that we are running 2 pretty different versions of gitlab-runner:

Large Richard:

• gitlab-runner: 11.3.0
• docker-ce: 18.06.1ce3-0~debian

Whitewhale:

• gitlgitlab-ci-multi-runner: 9.5.1
• docker-ce: ??

I don't think the docker version matters a lot, but I'm not sure having a 2 major Gitlab version difference between the runners is a good idea.

I'm using the upstream Gitlab and Docker packages since the Debian ones are lacking... I'm also upgrading them automatically with unattended-upgrades.