Merge branch 'make-gitlab-select-a-suitable-runner' into 'develop'

Make GitLab CI choose the right build runner

See merge request opensource/PrivateStorageio!78

.gitlab-ci.yml

+default:
+  # Guide the choice of an appropriate runner for all these jobs.
+  # https://docs.gitlab.com/ee/ci/runners/#runner-runs-only-tagged-jobs
+  tags:
+    - "nixos"
+    - "shell"
+
 docs:
   stage: "build"
   script: