Docker's logic for resolving credentials from .dockercfg accepts two kinds of matches:
1. an exact match between the dockercfg entry and the image prefix
2. a hostname match between the dockercfg entry and the image prefix
This change implements the latter, which permits the docker client to take .dockercfg entries of the form:
https://quay.io/v1/
and use them for images of the form:
quay.io/foo/bar
even though they are not a prefix-match.
2d396797a9