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).
This will make sure "variant" gets set properly on the resulting images if our Docker version is 20.10+.
We set this as tightly/sparingly as possible because the `--platform` flag is used for several different things inside Docker, the most visible being platform mismatch warnings/errors that we want to avoid.
Tianon Gravi