Dockerfile-test: upgrade Go version to 1.9.1

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

Dockerfile-test

@@ -1,4 +1,4 @@
-FROM golang:1.9-stretch
+FROM golang:1.9.1-stretch
 
 RUN apt-get -y update
 RUN apt-get -y install \