[ECR Public] [request]: cross-repository blob mounts failing?

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**Tell us about your request**
I filed an issue on @estesp's `manifest-tool` at https://github.com/estesp/manifest-tool/issues/124, but in testing further, even `docker manifest` fails to successfully create the cross-repository manifest lists, so I believe this is something deeper.

As seen in my latest comment there, the short version is that:

`docker manifest create public.ecr.aws/debian/debian:bullseye public.ecr.aws/debian/amd64:bullseye public.ecr.aws/debian/armel:bullseye public.ecr.aws/debian/armhf:bullseye public.ecr.aws/debian/arm64:bullseye public.ecr.aws/debian/i386:bullseye public.ecr.aws/debian/mips64el:bullseye public.ecr.aws/debian/ppc64el:bullseye public.ecr.aws/debian/s390x:bullseye`

followed by:

`docker manifest push public.ecr.aws/debian/debian:bullseye`

leads to:

`error mounting debian/amd64@sha256:4c0bfd2a9f4e90b5a07958f07d71b2e2ea5a965649c92bf35822aece72a29c6b to public.ecr.aws/debian/debian:bullseye`

**Which service(s) is this request for?**
ECR Public

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
At a high level, I have per-architecture images pushed to each of `public.ecr.aws/debian/amd64`, `public.ecr.aws/debian/armel`, etc, and I want to push the combined manifest lists to `public.ecr.aws/debian/debian`. Functionally, this means the clients creating the manifest list have to do [cross-repository blob mounts](https://github.com/opencontainers/distribution-spec/blob/v1.0.0/spec.md#mounting-a-blob-from-another-repository), but for some reason that doesn't seem to be working properly in any of the three tools I've tested (which I use to do exactly this successfully against other registries).

**Are you currently working around this issue?**
I was hoping `docker manifest` would be my workaround, but it looks like I'm going to have to go as far as using containerd to do push-by-digest of all the per-arch images instead. :disappointed:

cc @samuelkarp

Start by reproducing the listed docker manifest create and docker manifest push commands against ECR Public, then compare the behavior with manifest-tool issue #124 and the OCI distribution-spec blob-mounting section. Done means cross-repository blob mounts work when combining the per-architecture repositories into the target manifest list without the reported mounting error.

Written by the indexing model from the issue text.