Modify alpine version to 3.9 for supporting Arm64

Modify the alpine version to 3.9 for supporting arm64

Change-Id: I727d1264a90b53dc728a02e983690060ae68a754
Signed-off-by: Jingzhao.Ni <Jingzhao.Ni@arm.com>
diff --git a/Dockerfile b/Dockerfile
index a8abc63..6b3b440 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -22,7 +22,8 @@
 ADD . /go/src/gerrit.opencord.org/sadis-server
 RUN CGO_ENABLED=0 GOOS=linux go build -o /build/entry-point gerrit.opencord.org/sadis-server
 
-FROM alpine:3.5
+# The amd64 and arm64 are supported by alpine:3.6.
+FROM alpine:3.9
 MAINTAINER Open Networking Foundation <info@opennetworking.org>
 
 COPY --from=builder /build/entry-point /service/entry-point