Add docs for gitlab-runner issue 1042 (FF_NETWORK_PER_BUILD)

What does this MR do?

This MR adds documentation about the feature flag FF_NETWORK_PER_BUILD introduced in gitlab-runner#1042 (closed) in the services section.

This MR reopens !43875 (closed). The original Merge Request was closed by the author because it seemed to have been abandoned by the reviewers while discussing if the content should (additionally?) be added elsewhere.

I am re-adding these changes again As mentioned in the original discussion even tough the feature is partially documented is is practically impossible for a developer to discover for it while searching for how to enable inter-container network access during a build. In my humble opinion, since there exist different use cases for this feature, adding specific examples immediately adds benefit to developers looking to network connect their service containers during CI runs.

Under Gitlab's own Minimal Viable Merge Request rule future MRs can still improve upon or move this content elsewhere.

Thanks again to @Progdrasil for writing the original version of this MR and giving permission to reopen it.

Best Regards @systemkern

Screenshots (strongly suggested)

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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