refactor: use 'default.nix' for all modules

modules/docker/default.nix

@@ -0,0 +1,44 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.docker;
+in
+{
+  options.docker = {
+    rootless = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+    };
+    user = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+    };
+  };
+  config = lib.mkMerge [
+    {
+      warnings = lib.flatten [
+        (lib.optional (
+          cfg.rootless && cfg.user != null
+        ) "'virtualisation.docker.user' is ignored when rootless mode is enabled")
+        (lib.optional (
+          !cfg.rootless && cfg.user == null
+        ) "'virtualisation.docker.user' is not set (no user is added to the docker group)")
+      ];
+    }
+    (lib.mkIf cfg.rootless {
+      virtualisation.docker = {
+        enable = false;
+        rootless = {
+          enable = true;
+          setSocketVariable = true;
+        };
+      };
+    })
+    (lib.mkIf (!cfg.rootless && cfg.user != null) {
+      virtualisation.docker = {
+        enable = true;
+      };
+      users.users.${cfg.user}.extraGroups = [ "docker" ];
+    })
+  ];
+}