[Dockerfile] add new alpine build stage

Dockerfile

 #
 # build container
 #
-FROM golang:1-alpine
+FROM golang:1-alpine as builder
 WORKDIR /go/src/github.com/oliver006/redis_exporter/
 ADD main.go /go/src/github.com/oliver006/redis_exporter/
@@ -20,15 +20,25 @@ RUN CGO_ENABLED=0 GOOS=linux go build -o /redis_exporter \
     -ldflags  "-s -w -extldflags \"-static\" -X main.VERSION=$TAG -X main.COMMIT_SHA1=$SHA1 -X main.BUILD_DATE=$DATE" .
+#
+# Alpine release container
+#
+FROM alpine as alpine
+
+COPY --from=builder /redis_exporter /redis_exporter
+COPY --from=builder /etc/ssl/certs /etc/ssl/certs
+
+EXPOSE     9121
+ENTRYPOINT [ "/redis_exporter" ]
 #
 # release container
 #
-FROM scratch
-COPY --from=0 /redis_exporter /redis_exporter
-COPY --from=0 /etc/ssl/certs /etc/ssl/certs
+FROM scratch as scratch
+COPY --from=builder /redis_exporter /redis_exporter
+COPY --from=builder /etc/ssl/certs /etc/ssl/certs
 EXPOSE     9121
 ENTRYPOINT [ "/redis_exporter" ]