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/server/default.nix

@@ -0,0 +1,57 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.services.drone;
+in
+{
+  config = lib.mkIf cfg.enable {
+    systemd.services.drone-server = {
+      wantedBy = [ "multi-user.target" ];
+      after = [ "postgresql.service" ];
+      requires = [ "postgresql.service" ];
+      serviceConfig = {
+        EnvironmentFile = [
+          cfg.secretFile
+          cfg.sharedSecretFile
+        ];
+        Environment = [
+          "DRONE_DATABASE_DATASOURCE=postgres:///drone?host=/run/postgresql"
+          "DRONE_SERVER_HOST=drone.${config.networking.domain}"
+          "DRONE_SERVER_PROTO=https"
+          "DRONE_DATABASE_DRIVER=postgres"
+          "DRONE_SERVER_PORT=:${toString cfg.port}"
+          "DRONE_USER_CREATE=username:${cfg.admin},admin:true"
+          "DRONE_JSONNET_ENABLED=true"
+          "DRONE_STARLARK_ENABLED=true"
+        ];
+        ExecStart = "${pkgs.drone}/bin/drone-server";
+        User = "drone";
+        Group = "drone";
+      };
+    };
+
+    users.users.drone = {
+      isSystemUser = true;
+      createHome = true;
+      group = "drone";
+    };
+    users.groups.drone = { };
+
+    services.postgresql = {
+      enable = true;
+      ensureDatabases = [ "drone" ];
+      ensureUsers = [{
+        name = "drone";
+        ensurePermissions = {
+          "DATABASE drone" = "ALL PRIVILEGES";
+        };
+      }];
+    };
+
+    my.services.nginx.virtualHosts = [
+      {
+        subdomain = "drone";
+        inherit (cfg) port;
+      }
+    ];
+  };
+}