This command will, given a remote image reference, look up the list of platforms from it and match them to supported bashbrew architectures (providing content descriptors for each).
Also, refactor registry code to be more correct: previously, this couldn't fetch from Docker without `DOCKERHUB_PUBLIC_PROXY` (see `registry-1.docker.io` change) and was ignoring content digests. Now it works correctly with or without `DOCKERHUB_PUBLIC_PROXY`, verifies the size of every object it pulls, verifies the digest, _and_ should continue working with the in-progress Moby containerd-integration (where the local image ID becomes the digest of the manifest or index instead of the digest of the config blob as it is today).
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