add git submodule ignore code and notice in nix derivation

Author: juliankuners

{{cookiecutter.project_slug}}/README.md

@@ -20,3 +20,6 @@ Use `make` to run common tasks (see the [Makefile](Makefile) for a complete list
 * `make format`: Format code
 * `make test-unit`: Run unit tests
 * `make test-integration`: Run integration tests
+
+## Considerations
+If you use git submodules that are not required for building the project with `nix`, then you should add these directories to the `gitignoreSourcePure` list in `nix/default.nix`, see the respective left-over `TODO` in the nix file. Otherwise, the nix build that is uploaded to the nix binary cache by CI might not match the version that is requested by `kup`, in case this project is offered as a package by `kup`. This is due to weird behaviour in regards to git submodules by `git` and `nix`, where a submodule directory might exist and be empty or instead not exist, which impacts the resulting nix hash, which is of impartance when offering/downloading cached nix derivations.

{{cookiecutter.project_slug}}/nix/{{cookiecutter.project_slug}}/default.nix

@@ -1,6 +1,7 @@
 {
   lib,
   callPackage,
+  nix-gitignore,
 
   pyproject-nix,
   pyproject-build-systems,
@@ -16,7 +17,17 @@ let
 
   # load a uv workspace from a workspace root
   workspace = uv2nix.lib.workspace.loadWorkspace {
-    workspaceRoot = ../..;
+    workspaceRoot = lib.cleanSource (nix-gitignore.gitignoreSourcePure [
+        ../../.gitignore
+        ".github/"
+        "result*"
+        # do not include submodule directories that might be initilized empty or non-existent due to nix/git
+        # otherwise cachix build might not match the version that is requested by `kup`
+        # TODO: for new projects, add your submodule directories that are not required for nix builds here!
+        # e.g., `"/docs/external-computation"` with `external-computation` being a git submodule directory
+        # "/docs/external-computation"
+      ] ../..
+    );
   };
 
   # create overlay