Improve Shared Runners message and make it more descriptive for GitLab.com
I think that current message is not clear enough:
Shared runners execute code of different projects on the same Runner unless you configure GitLab Runner Autoscale with MaxBuilds 1 (which it is on GitLab.com).
We should improve the default. Maybe we should say something like that:
Your shared runners may or may not run your builds securely. It depends on the configuration of Shared Runner and chosen way of executing your builds. Ask your Administrator how secure are your builds.
In case of GitLab.com we can use this feature specifically for this: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3750. I think that we should say something like this (with better wording):
We try to ensure that your builds are secure. Your project is run only once on virtual machine. Shared Runners are sponsored by Digital Ocean. You can read more about our shared runners configuration by visiting our blog post: https://about.gitlab.com/2016/04/05/shared-runners/.
What do you think? @axil @sytses @markpundsack @grzesiek @ashleys