gitlab-runner command to pre-pull helper image
Description
The Windows Docker Executor helper image for 1809 takes on average 1.9 minutes to pull on our Azure VMs, just barely making it under the 2 minute timeout. Any hiccups will cause a job to fail when the first job is run on a new machine.
Proposal
Since the "default" helper image is undiscoverable via any automation, gitlab-runner should expose a command to docker pull the helper image so I can initiate this prior to marking the runner ready, saving users from failed jobs on new runners. This should be built-in to gitlab-runner, because the helper image is not exposed to automation in any way.
An alternate solution would be a command that returned the helper image value, so we could discover the default helper image and then docker pull it ourselves.