Refactor multi-arch support (especially for new mostly-official multi-arch official images)

See https://github.com/docker-library/official-images/blob/99433d2ca217bd9479169c93a20b6f6530f94856/README.md#architectures-other-than-amd64 for some context of where these images come from.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>