Add garnix CI cache
diff --git a/modules/cache.nix b/modules/cache.nix
new file mode 100644
index 0000000..eee9d79
--- /dev/null
+++ b/modules/cache.nix
@@ -0,0 +1,8 @@
+{
+  nix.settings = {
+    substituters = [ "https://cache.garnix.io" ];
+    trusted-public-keys = [
+      "cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g="
+    ];
+  };
+}