[tor-bugs] #22891 [Core Tor/Tor]: Add GitLab CI configs

[Tor Bug Tracker & Wiki]

#22891: Add GitLab CI configs
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     Reporter:           |      Owner:
  catalyst               |
         Type:  defect   |     Status:  new
     Priority:  High     |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core     |    Version:
  Tor/Tor                |   Keywords:  ci continuous-integration testing
     Severity:  Normal   |  best-practice unit-testing new-developers
Actual Points:           |  Parent ID:
       Points:           |   Reviewer:
      Sponsor:           |
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 We'll soon have Travis configs (#22636), so we should also add a `.gitlab-
 ci.yml` or something so GitLab's continuous integration will do automated
 builds for oniongit and gitlab.com forks (and merge requests).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22891>
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