[RELEASE] Bump bk version to 4.7.1 in docker file

Descriptions of the changes in this PR:

Bump bk version to 4.7.1 to build docker image for `4.7.1`

Author: Sijie Guo <sijie@apache.org>

Reviewers: Jia Zhai <None>

This closes apache#1523 from sijie/bump_docker_version

docker/Dockerfile

@@ -20,7 +20,7 @@
 FROM centos:7
 MAINTAINER Apache BookKeeper <dev@bookkeeper.apache.org>
 
-ARG BK_VERSION=4.7.0
+ARG BK_VERSION=4.7.1
 ARG DISTRO_NAME=bookkeeper-server-${BK_VERSION}-bin
 ARG GPG_KEY=FD74402C