The `--userland-proxy` daemon flag makes it possible to rely on hairpin
NAT and additional iptables routes instead of userland proxy for port
publishing and inter-container communication.
Usage of the userland proxy remains the default as hairpin NAT is
unsupported by older kernels.
Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
ee666e8d4b