Replace docker.io with index.docker.io in ImageID

[samb1729]

Requests to docker.io redirect to www.docker.com rather than index.docker.io, so this change will rewrite image ids to use the latter.

Fixes #692

[squaremo]

It's fine and good this got merged; it made me wonder if there's not a better overall approach to dealing with image names vs where to fetch image manifests.

At the minute we essentially mutate the image name to its canonical form. But this means, for example, that the image names we display in list-images aren't always as supplied by the user, and that can be confusing and might even lead to mistaken copypasta.

So I think it'd be better to keep image names as-is in results, and just use the canonical form where it's needed for fetching things.

[squaremo]

Created #704