Build: Fix Dockerfile glibc version error (grafana#19394)

Change the base image for the golang build stage from 
golang:1.12.9 (Debian 'Buster' 10) to golang:1.12.9-stretch 
(Debian 'Stretch' 9), so that the glibc version in golang build 
stage matches the glibc version in the final container based 
on ubuntu:18.04. Otherwise, the built binaries cannot execute 
in the final container.

Dockerfile

@@ -1,5 +1,5 @@
 # Golang build container
-FROM golang:1.12.9
+FROM golang:1.12.9-stretch
 
 WORKDIR $GOPATH/src/github.com/grafana/grafana