Add GitLab CI config

Not doing artifacts as of now since they are weird in GitLab CI and screw up NixOs building

.gitlab-ci.yml

+docs:
+  stage: build
+  script:
+    - nix-shell --run 'nix-build docs.nix'
+
+unit-tests:
+  stage: test
+  script:
+    - nix-shell --run 'nix-build nixos/unit-tests.nix' && cat result
+
+system-tests:
+  stage: test
+  timeout: 3 hours
+  script:
+    - nix-shell --run 'nix-build nixos/system-tests.nix'
+