doc: add README.md

README.md

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+This repository is just a for auto updating the default `nix` docker image for forgejo runner on my instance.
+
+# Setup:
+As you cannot run a forgejo runner with `nixos/nix` due to it not having `node` by default, you need to manually build and upload the first image. After that you can specify your instance's image as a base image for forgejo runner.
+
+### Manual upload of the first image:
+```bash
+  USERNAME="tamipes"
+  URL="git.tami.moe"
+  nix build --extra-experimental-features "flakes nix-command" .
+  docker login "https://$URL"
+  docker load -i result
+  docker image tag nix "$URL/$USERNAME/nix:latest"
+  docker push "$URL/$username/nix"
+
+  # I also tag with the date, but this might be unneeded
+  docker image tag nix "$URL/$USERNAME/nix:latest$(date --iso-8601)"
+  docker push "$URL/$username/nix"
+```
+
+
+# Extra info
+- This image is essentially still "based" on `nixos/nix`, but it builds it with nodejs included. Checkout the flake.nix!
+  - it does not use `nixos/nix` as a base image, but uses the same build script and adds extra stuff
+  - [nixpkgs manual](https://nix.dev/manual/nix/2.24/installation/installing-docker#docker-image-with-the-latest-development-version-of-nix)
+  - [nixos/nix build script](https://github.com/NixOS/nix/blob/master/docker.nix)