Rather than re-execing docker as the proxy, create a new command docker-proxy
that is much smaller to save memory in the case where there are a lot of
procies being created. Also allows the proxy to be replaced, for example
in Docker for Mac we have a proxy that proxies to osx instead of locally.
This is the vendoring pull for https://github.com/docker/docker/pull/23312
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
