Add gitlab-ci configuration.

.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"
+
+build:
+  stage: build
+  script:
+    - "nix build -Lv -f nix"