Dockerfile: Update to Go 1.12.6, matching moby/moby

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

libnetwork/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.10.7 as dev
+FROM golang:1.12.6 as dev
 RUN apt-get update && apt-get -y install iptables \
 		protobuf-compiler