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/woodpecker/default.nix

@@ -0,0 +1,46 @@
+{ lib, ... }:
+{
+  imports = [
+    ./agent-docker
+    ./agent-exec
+    ./server
+  ];
+
+  options.my.services.woodpecker = with lib; {
+    enable = mkEnableOption "Woodpecker 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 Woodpecker UI";
+    };
+    rpcPort = mkOption {
+      type = types.port;
+      default = 3031;
+      example = 8080;
+      description = "Internal port of the Woodpecker UI";
+    };
+    secretFile = mkOption {
+      type = types.str;
+      example = "/run/secrets/woodpecker.env";
+      description = "Secrets to inject into Woodpecker server";
+    };
+    sharedSecretFile = mkOption {
+      type = types.str;
+      example = "/run/secrets/woodpecker.env";
+      description = "Shared RPC secret to inject into server and runners";
+    };
+  };
+}