Support image registry mirrors/pull-through caches

[etungsten]

What I'd like:
A way to configure image pull-through caches/registry mirrors for the container runtimes.

A generic setting of the following form would probably work:

[settings.image-registry.mirrors]
"docker.io" = ["my-own-docker-registry-cache", "some-other-docker-registry", "official-docker-registry"]
...

Note that docker only supports mirroring/caching with DockerHub. So we might need to special case the template for daemon.json like so:

{{~#if settings.image-registry.mirrors."docker.io"}}
"registry-mirrors": [{{join_array ", " settings.image-registry.mirrors."docker.io"}}]
{{~/if}}

For cri-containerd, we'd configure it here.

Any alternatives you've considered:
None