tests: add imagetools tests for copying manifests and indexes

[jedevc]

While in the process of investigating, #1660, realized we didn't have any tests for it.

I've added some tests to ensure that manifests and indexes are copied with their digests preserved intact.

[jedevc]

Out of curiosity: is this case actually correct? If the imagetools create input is only a single manifest, do we expect to create an index containing just it? Or would we expect to just copy it unchanged?

[crazy-max]

Hum I would say we just copy it unchanged imo. It looks linked to #1797 (comment) where we default to OCI now?

[jedevc]

Hm, I don't think this is oci/docker media type confusion, it seems like this was always present before that PR?

I think maybe we want the just copy behaviour agreed, I think it's worth merging this PR to track the current behaviour, then fixing that as a followup, if we agree that's worth doing?

[crazy-max]

Hm, I don't think this is oci/docker media type confusion, it seems like this was always present before that PR?

Ah right indeed this is for manifest list / index case.

I think maybe we want the just copy behaviour agreed,

I took a further look and we already return the original manifest for a single source:

buildx/util/imagetools/create.go

Lines 79 to 84 in 296832c

	// on single source, return original bytes
	if len(srcs) == 1 && len(ann) == 0 {
	if mt := srcs[0].Desc.MediaType; mt == images.MediaTypeDockerSchema2ManifestList || mt == ocispec.MediaTypeImageIndex {
	return dts[0], srcs[0].Desc, nil
	}
	}

But seems we hit this case instead:

buildx/util/imagetools/create.go

Lines 115 to 121 in 296832c

	var mfst ocispec.Index
	if err := json.Unmarshal(dts[i], &mfst); err != nil {
	return nil, ocispec.Descriptor{}, errors.WithStack(err)
	}
	for _, d := range mfst.Manifests {
	addDesc(d)
	}

I think it's worth merging this PR to track the current behaviour, then fixing that as a followup, if we agree that's worth doing?

Yes sounds good for a follow-up 👍