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/agent-exec/default.nix

@@ -0,0 +1,64 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.services.woodpecker;
+
+  hasRunner = (name: builtins.elem name cfg.runners);
+in
+{
+  config = lib.mkIf (cfg.enable && hasRunner "exec") {
+    services.woodpecker-agents = {
+      agents.exec = {
+        enable = true;
+
+        environment = {
+          WOODPECKER_SERVER = "localhost:${toString cfg.rpcPort}";
+          WOODPECKER_MAX_WORKFLOWS = "10";
+          WOODPECKER_BACKEND = "local";
+          WOODPECKER_FILTER_LABELS = "type=exec";
+          WOODPECKER_HEALTHCHECK = "false";
+
+          NIX_REMOTE = "daemon";
+          PAGER = "cat";
+        };
+
+        path = with pkgs; [
+          woodpecker-plugin-git
+          bash
+          coreutils
+          git
+          git-lfs
+          gnutar
+          gzip
+          nix
+        ];
+
+        environmentFile = [ cfg.sharedSecretFile ];
+      };
+    };
+
+    # Adjust runner service for nix usage
+    systemd.services.woodpecker-agent-exec = {
+      # Might break deployment
+      restartIfChanged = false;
+
+      serviceConfig = {
+        # Same option as upstream, without @setuid
+        SystemCallFilter = lib.mkForce "~@clock @privileged @cpu-emulation @debug @keyring @module @mount @obsolete @raw-io @reboot @swap";
+
+        BindPaths = [
+          "/nix/var/nix/daemon-socket/socket"
+          "/run/nscd/socket"
+        ];
+        BindReadOnlyPaths = [
+          "/etc/passwd:/etc/passwd"
+          "/etc/group:/etc/group"
+          "/etc/nix:/etc/nix"
+          "${config.environment.etc."ssh/ssh_known_hosts".source}:/etc/ssh/ssh_known_hosts"
+          "/etc/machine-id"
+          # channels are dynamic paths in the nix store, therefore we need to bind mount the whole thing
+          "/nix/"
+        ];
+      };
+    };
+  };
+}