How to self-host Forgejo runner with (Nix) cache?

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I’m happily self-hosting Forgejo (~ Gitea) and a working local runner on a NixOS server. I only can’t get caching to work (to avoid spamming cache.nixos.org with requests). I’m currently trying to get (Github) actions caching to work like this

on: [push]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - run: echo All good!
      # v5 only works with newer runner tool act, when Forgejo updates to it
      - uses: actions/checkout@v4
            - name: Install Nix
      - uses: cachix/install-nix-action@v31
      - name: Nix cache
        uses: actions/cache@v3
        with:
          path: /nix/store
          key: nix-cache
      # - uses: DeterminateSystems/magic-nix-cache-action@main
      - run: nix build .

In the job cache steps i always get an error:

::warning::Failed to save: reserveCache failed: connect ETIMEDOUT 10.0.0.3:34783
::warning::Cache save failed.

I’m not sure if i have to host a separate “Caching server” or just fix some runner settings. My settings are:

services.gitea-actions-runner = {
    package = pkgs.forgejo-actions-runner;
    instances.default = {
      enable = true;
      ....
      labels = [
        # Built similar to Github images but for ACT (forgejo's runner tool)
        # and can install Nix and evaluate. Recommended by ACT:
        # https://github.com/catthehacker/docker_images
        "ubuntu-latest:docker://ghcr.io/catthehacker/ubuntu:act-24.04"
      ];
      # https://gitea.com/gitea/act_runner/src/branch/main/internal/pkg/config/config.example.yaml#L50
      settings = {
        cache.host = "localhost";
        # github actions seem to want to use this port, but doesn't fix it.
        cache.port = 34783;
      };
    };
  };

How do you self-host runners with cache? Is the Github actions approach actually the best way, or should i rather host a Nix cache server like Attic.rs? If so, do you have examples?

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