Build: Upgrade to go 1.12.9 (#18638)

* Build: Upgrade to go 1.12.9

* Build: Use default-mysql-client for debian buster

The go base image has been updated to use Debian Buster instead of
Stretch, which seems to have done away with mysql-client in favor of
default-mysql-client.

* Build: Update Dockerfile to use go 1.12.9

Fixes #18592

scripts/build/ci-build/Dockerfile

@@ -71,7 +71,7 @@ RUN apt-get update   && \
 # base image to crossbuild grafana
 FROM ubuntu:14.04
 
-ENV GOVERSION=1.12.6 \
+ENV GOVERSION=1.12.9 \
     PATH=/usr/local/go/bin:$PATH \
     GOPATH=/go \
     NODEVERSION=10.14.2

scripts/build/ci-build/build-deploy.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-_version="1.2.7"
+_version="1.2.8"
 _tag="grafana/build-container:${_version}"
 
 docker build -t $_tag .