Dockerfile.prod: use proxy.golang.org

Also update builder to Go 1.12

Change-Id: I87b25aa9cdd826c77ff71f6dc6166d868642ea19
Reviewed-on: https://go-review.googlesource.com/c/website/+/178238
Reviewed-by: Katie Hockman <[email protected]>

Author: andybons

cmd/golangorg/Dockerfile.prod

@@ -1,7 +1,7 @@
 # Builder
 #########
 
-FROM golang:1.11 AS build
+FROM golang:1.12 AS build
 
 RUN apt-get update && apt-get install -y \
       zip # required for generate-index.bash
@@ -16,6 +16,7 @@ RUN cd /goroot/src && ./make.bash
 ENV GOROOT /goroot
 ENV PATH=/goroot/bin:$PATH
 ENV GO111MODULE=on
+ENV GOPROXY=https://proxy.golang.org
 
 RUN go version