docker: loop auth handlers instead of using just one
docker: loop auth handlers instead of using just one

As already done docker-side at
https://github.com/moby/moby/blob/master/vendor/github.com/docker/distribution/registry/client/auth/session.go#L98

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

containers/image,masters-of-cats/image,containers/image