Add Gitlab CI config

.gitlab-ci.yml

+stages:
+- setup
+- triggers
+
+# Since the gitlab-ci.yml format is relatively limited, we generate the nixos-rebuild jobs as a child pipeline
+# See https://about.gitlab.com/blog/2020/04/24/parent-child-pipelines/ for more info on child pipelines
+generate-nixos-pipeline:
+  stage: setup
+  tags:
+  - operations-nix
+  script:
+  - nix-build .nixos-gitlab-ci.yml.nix
+  - cp result .nixos-gitlab-ci.yml
+  artifacts:
+    paths:
+    - .nixos-gitlab-ci.yml
+
+nixos:
+  stage: triggers
+  needs:
+  - generate-nixos-pipeline
+  trigger:
+    include:
+    - artifact: .nixos-gitlab-ci.yml
+      job: generate-nixos-pipeline
+    strategy: depend
+
+editorconfig:
+  stage: triggers
+  needs: []
+  tags:
+  - operations-nix
+  script:
+  - nix-shell -p editorconfig-checker --run editorconfig-checker -I nixpkgs=channel:nixos-unstable
+
+iso_image:
+  stage: triggers
+  needs: []
+  tags:
+  - operations-nix
+  script:
+  - nix-build -A isoImage
+  artifacts:
+    paths:
+    - result/*
+
+kexec_tarball:
+  stage: triggers
+  needs: []
+  tags:
+  - operations-nix
+  script:
+  - nix-build -A kexec_tarball
+  artifacts:
+    paths:
+    - result/*

.nixos-gitlab-ci.nix

+let
+  inherit (import ./.) hosts pkgs lib;
+
+  jobs = lib.mapAttrs (
+    name: host: {
+      stage = "build";
+      tags = [ "operations-nix" ];
+      variables.GIT_DEPTH = 0;
+      script = [
+        "nix-build --show-trace --pure-eval --option allow-import-from-derivation false --expr \"( (import (fetchGit { url = ./.; rev = \\\"$(git describe --always --abbrev=0)\\\"; })).deploy.${name} )\""
+      ];
+    }
+  ) (
+    lib.filterAttrs (
+      name: host: !host.config.skipCI
+    ) hosts
+  );
+
+in pkgs.writeText "nixos-gitlab-ci.yml" (builtins.toJSON jobs)