Merge branch 'gitlab-ci-4' into 'master'

Add GitLab CI config

See merge request opensource/PrivateStorageio!61

.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'"
+