Dev Docker: Use golang:1.12.9-alpine to prevent glibc mismatch. (#18701)

Using golang:1.12.9 which is based on Debian as a build container
leads to a version mismatch for glibc when running grafana-server
in alpine:3.10

grafana-server: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by grafana-server)

Dockerfile

@@ -1,5 +1,7 @@
 # Golang build container
-FROM golang:1.12.9
+FROM golang:1.12.9-alpine
+
+RUN apk add --no-cache gcc g++
 
 WORKDIR $GOPATH/src/github.com/grafana/grafana