Pass authentication credentials through to build

In Docker 1.10 and earlier, "docker build" can do a build FROM a private
repository that hasn't yet been pulled. This doesn't work on master. I
bisected this to https://github.com/docker/docker/pull/19414.
AuthConfigs is deserialized from the HTTP request, but not included in
the builder options.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>