The CI experience when working with forks could be improved
If a source repository has CI enabled, and one or more users fork the project, the fork is responsible for configuring their own CI. When using shared runners, the burden is somewhat reduced, but it would be reduced further if any merge request from a fork to the source would be built on the source repository's runner.
This is somewhat related to https://gitlab.com/gitlab-org/gitlab-ce/issues/4176, but the conversation on that issue has diverged slightly. This is not related to testing changes when merged with master, specifically, it's about improving the fork testing flow.
When a fork opens a merge request against the source repository, issue the build against the source repository's runners. This ensures the fork does not need to set up CI. It lowers to barrier to contributions. It also affects customers that use a fork workflow, as reported in https://gitlab.zendesk.com/agent/tickets/37544.