Install GitLab Runner as a separate layer for caching

What does this merge request do and why?

Attempt to reduce the amount of memory needed to build Gitpod image with podman. A patch from !2782 (closed).

The MR removes GITLAB_RUNNER_DISABLE_SKEL=true which is the default since 14.0.

I am also leaving apt cache, because if it is not cleaned up immediately, it will increase the size of image anyway. And if is it removed at this point, then support/bootstrap-packages will have to redownload it, which will increase image build time. I believe the build time is more critical here.

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.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.