Bump the GO Version to 1.12.8 which contains security fixes

https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/golang-announce/65QixT3tcmg/DrFiG6vvCwAJ

Also pin to -stretch

Signed-off-by: Arko Dasgupta <arko.dasgupta@docker.com>

libnetwork/Dockerfile

@@ -1,6 +1,6 @@
-ARG GO_VERSION=1.12.6
+ARG GO_VERSION=1.12.8
 
-FROM golang:${GO_VERSION} as dev
+FROM golang:${GO_VERSION}-stretch as dev
 RUN apt-get update && apt-get -y install iptables \
 		protobuf-compiler