Add docker pull option for runner docker

What does this merge request do and why?

Add docker pull option for runner docker

This ensures that Docker will always run the latest image for gitlab/gitlab-runner:latest that is publicly available. The default option for docker --pull is missing which means that the first time docker run gitlab/gitlab-runner:latest is run, it only pulls the latest image at that time. Subsequent docker run will continue using the same image, even if a newer one is available.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.