Fix gitlab-qa image with mirror

This fixes the gitlab-qa image temporarily (see #28 (closed)) by using a mirror.

Before I used the mirror, I have executed:

docker run -it --rm registry.gitlab.com/gitlab-org/gitlab-build-images:gitlab-qa sha256sum /usr/local/bin/docker

to get the sha256 sum and be sure that noone has tempered with the docker binary at the mirror.