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>
Upstream-commit: 6fed46aeb97943315aed12f2dc62565f7bcc53dc
Component: engine

components/engine/api/server/router/build/build_routes.go

@@ -159,6 +159,8 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 		buildOptions.Dockerfile = dockerfileName
 	}
 
+	buildOptions.AuthConfigs = authConfigs
+
 	out = output
 	if buildOptions.SuppressOutput {
 		out = notVerboseBuffer