Add comments to Dockerfile

Author: ProgEsteves

Dockerfile

@@ -1,10 +1,13 @@
-FROM quay.io/prometheus/golang-builder as builder
+FROM quay.io/prometheus/golang-builder AS builder
 
+# Get sql_exporter
 ADD .   /go/src/github.com/free/sql_exporter
 WORKDIR /go/src/github.com/free/sql_exporter
 
+# Do makefile
 RUN make
 
+# Make image and copy build sql_exporter
 FROM        quay.io/prometheus/busybox:glibc
 MAINTAINER  The Prometheus Authors <[email protected]>
 COPY        --from=builder /go/src/github.com/free/sql_exporter/sql_exporter  /bin/sql_exporter