chore(rootfs/Dockerfile): update go toolchain to 1.12 (#152)

See https://golang.org/doc/go1.12

Author: mboersma

rootfs/Dockerfile

@@ -3,7 +3,7 @@ FROM ubuntu:16.04
 LABEL name="deis-go-dev"
 
 ENV AZCLI_VERSION=2.0.46 \
-    GO_VERSION=1.11.5 \
+    GO_VERSION=1.12 \
     GLIDE_VERSION=v0.13.1 \
     GLIDE_HOME=/root \
     HELM_VERSION=v2.12.3 \