Dockerfile.prod: use proxy.golang.org
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 <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website