Describe available platforms when no match

[stepancheg]

Tested with skopeo:

$ skopeo --insecure-policy copy --override-os=linux --override-arch=x86_64 docker://alpine dir:dir

FATA[0000] choosing an image from manifest list docker://alpine:latest: no image found in image index for architecture "x86_64", variant "", OS "linux"; available platforms: arch="amd64" os="linux" variant="", arch="unknown" os="unknown" variant="", arch="arm" os="linux" variant="v6", arch="unknown" os="unknown" variant="", arch="arm" os="linux" variant="v7", arch="unknown" os="unknown" variant="", arch="arm64" os="linux" variant="v8", arch="unknown" os="unknown" variant="", arch="386" os="linux" variant="", arch="unknown" os="unknown" variant="", arch="ppc64le" os="linux" variant="", arch="unknown" os="unknown" variant="", arch="riscv64" os="linux" variant="", arch="unknown" os="unknown" variant="", arch="s390x" os="linux" variant="", arch="unknown" os="unknown" variant=""

helped me to figure out expected spelling for x86_64 is amd64.

[mtrmac]

Thanks!

• If this should be done, it must be consistent between OCI indices and schema2 lists. It’s about the same code, anyway.
• There can be multiple entries with the same platform (but differently-compressed), it would be nice not to duplicate those
• There already is some similar code in prepareImageConfigForDest; at least some part of that can almost certainly be shared across the three locations, in some internal subpackage.
• (And then there’s the … fun … part about designing an error message that is still readable; I suppose trying to fit it into 80 columns is a lost cause, but, well, 800 might be too much. The error message should make sense for the errors encountered by a typical user, and I’d argue that “spelling for x86_64” is a fairly rare failure — most users build images using existing tool that handle this. That’s not to say that the motivation should be discarded, but maybe eventually users could use future containers/skopeo#1283 instead of seeing everything directly inside the error message. There might not be any obviously perfect solution and having an imperfect one would still be an improvement — just, it’s something that deserves a bit of thought.)