feat: nix buildpack

[olevski]

This works. The only thing left to do is that there is an order and grouping to image extensions. And by adding the 2nd one that installs nix in a build image if I flip the image extension order things break. I will check if you can have 2 groups of image extensions, rather than 1.

The buildpack works by:

• installing nix in the build image (via an image extension)
• running nix build .... which places a bunch of symlinks to /nix/store in the buildpack layer
• follows the symlinks from above and copies the real files in the buildpack layer
• the image extension makes the run image have a symlink that makes /nix/store point to the buildpack layer with all the real files

So usually when you run nix it produces stuff that symlinks to /nix/store but in this case we flip that around. /nix/store in the run image is a symlink to the layer that contains the full store created from the buildpack. We cannot avoid /nix/store and symlinks because the exectuables from nix are hardcoded to expect stuff exclusively in /nix/store.

[gabyx]

suggestion:
I would use nix copy to only copy the derivation you build

1. build with nix build --json --print-out-paths ".#default" --json | jq ".[].outputs.out" or nix build --print-out-paths ".#my-package"
2. Then you copy the closure to a new location with nix copy /nix/store/... --to "$nix_layer/nix/store

[gabyx]

future: Also the attrset path in the installable .#default could be customized maybe ...: <flake-path>#<attribute>

• ./tools/nix#my-stuff or so...

maybe not so important....