Dockerfile: Update to Go 1.12.6, matching moby/moby
Dockerfile: Update to Go 1.12.6, matching moby/moby

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@docker.com>

mahak/libnetwork,docker/libnetwork,docker/libnetwork,mahak/libnetwork,mahak/libnetwork,docker/libnetwork