Certain load balancers, such as Amazon's Elastic Load Balancer, have a very
limited notion of health. While a properly configured and operational registry
should always return a 401 when hitting "/v2/", such load balancers cannot be
configured to treat this response code as healthy. This changeset makes "/"
always return a 200 response, unless the health checks have failed.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
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