nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/services/drone/default.nix

@@ -0,0 +1,44 @@
+# A docker-based CI/CD system
+#
+# Inspired by [1]
+# [1]: https://github.com/Mic92/dotfiles/blob/master/nixos/eve/modules/drone.nix
+{ lib, ... }:
+{
+  imports = [
+    ./runner-docker
+    ./runner-exec
+    ./server
+  ];
+
+  options.my.services.drone = with lib; {
+    enable = mkEnableOption "Drone CI";
+    runners = mkOption {
+      type = with types; listOf (enum [ "exec" "docker" ]);
+      default = [ ];
+      example = [ "exec" "docker" ];
+      description = "Types of runners to enable";
+    };
+    admin = mkOption {
+      type = types.str;
+      default = "ambroisie";
+      example = "admin";
+      description = "Name of the admin user";
+    };
+    port = mkOption {
+      type = types.port;
+      default = 3030;
+      example = 8080;
+      description = "Internal port of the Drone UI";
+    };
+    secretFile = mkOption {
+      type = types.str;
+      example = "/run/secrets/drone-gitea.env";
+      description = "Secrets to inject into Drone server";
+    };
+    sharedSecretFile = mkOption {
+      type = types.str;
+      example = "/run/secrets/drone-rpc.env";
+      description = "Shared RPC secret to inject into server and runners";
+    };
+  };
+}