Configure Shared Runners on Staging Ref

Set up instance runners on Staging Ref that will be available to all groups and projects in a GitLab instance.

• https://docs.gitlab.com/ee/ci/runners/runners_scope.html#shared-runners

Steps

• Research runner configurations. Analyse what's used on current Staging. Would autoscale be used in this case?
• Set up runner
• Verify that it works and quota CI/CD minutes is set up
• Explore if monitoring needs to be setup
• Add documentation about this setup