Add image indexes for multi-arch containers

[thefloweringash]

It wasn't obvious to me how this worked at all. I thought it was sufficient to put a list of image manifests into a single directory in index.json. However, it seems like this is a less formal grouping of images. A multi-arch image is instead a directory of a single manifest, where that manifest is an image index, and that image index has a list of image manifests.

---

I should try to share more code between the python scripts.

[thefloweringash]

I should try to share more code between the python scripts.

This is slightly harder than I expected, and there's about one function that's worth sharing. I think I'll leave this as is.

[thatsmydoing]

So this would be used as

oci.makeImageDirectory {
  name = "foo";
  manifests = [(oci.makeImageIndex {
    name = "myimage";
    images = [ myimage-x86_64 myimage-arm64 ];
  })];
}

?

I should try to share more code between the python scripts.

This is slightly harder than I expected, and there's about one function that's worth sharing. I think I'll leave this as is.

Maybe it makes sense to consolidate everything into a single python script (aside from the layer builder itself)?

[thefloweringash]

So this would be used as

oci.makeImageDirectory {
  name = "foo";
  manifests = [(oci.makeImageIndex {
    name = "myimage";
    images = [ myimage-x86_64 myimage-arm64 ];
  })];
}

?

Indeed. The associated pull request has a more complete example of building an image with multiple architectures:

let
  makeImage = name: systems: makeSpec:
     let
       images = lib.forEach systems (system:
         let deps = packagesBySystem."${system}"; in
         ociTools.makeImageManifest ({
           architecture = deps.pkgs.go.GOARCH;
           os = deps.pkgs.go.GOOS;
         } // makeSpec deps)
       );
     in
       ociTools.makeImageDirectory {
         inherit name;
         manifests = [
           (ociTools.makeImageIndex {
             inherit name images;
           })
         ];
       };
in # ...

I should try to share more code between the python scripts.

This is slightly harder than I expected, and there's about one function that's worth sharing. I think I'll leave this as is.

Maybe it makes sense to consolidate everything into a single python script (aside from the layer builder itself)?

I don't mind the scripts being distinct entry points, I find it fairly easy to reason about. I do think we could benefit from sharing some more code, maybe like an OCIBuilder that handles the file layout.