Add daemon flag to specify public registry mirrors

Adds support for a --registry-mirror=scheme://host[:port]
daemon flag. The flag may be present multiple times. If
provided, mirrors are prepended to the list of endpoints used
for image pull. Note that only mirrors of the public
index.docker.io registry are supported, and image/tag resolution
is still performed via the official index.

Docker-DCO-1.1-Signed-off-by: Tim Smith <timbot@google.com> (github: timbot)
When I read the whole thing, as a whole, this bit felt (and still does feel) like clarifying, elaboration, and not an example. That's why I suggested to go with `i.e.,`.

My reason for that is/was, the given two explanations (and not examples) of the precedent cover up almost all cases of what's trying to be said.

If everybody agrees that these are indeed examples out of many and not an effort to elaborate what's being said, yes to `e.g.,` (with a comma ;-)).