push: dedupe push handlers
push: dedupe push handlers

This allows to work around the containerd issue on
tracking push status on pushing manifest lists.

Signed-off-by: Tonis Tiigi <c2470c48b2d3312d61f94f18d3a1cd113d1915ad@gmail.com>
tonistiigi/buildkit_poc,tonistiigi/buildkit_poc