Merged requested to merge documentation/apt-pinning-for-debian-package into master
What does this MR do?
Since Debian Stretch debian repositories have a native
gitlab-ci-multi-runner package. And the official repository has the higher priority.
This MR adds the APT pinning configuration step to installation documentation which should prioritize our repository.
Inspired by @caldwell in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1605#note_14731303