go: simplify the vpnkit-userland-proxy Dockerfile
go: simplify the vpnkit-userland-proxy Dockerfile

There's no need to round-trip through the Makefile for a cross-compile
build when we can build in native Docker.

The new Dockerfile is based on the LinuxKit standard package one.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit