ISSUE apache#697: bump version to 4.6.0 in Dockerfile

Descriptions of the changes in this PR:

Since 4.6.0 released, bump version to 4.6.0 in Dockerfile to use the latest release

Master Issue: apache#697

Author: Jia Zhai <zhaijia@apache.org>

Reviewers: Enrico Olivelli <eolivelli@gmail.com>

This closes apache#918 from jiazhai/issue-697, closes apache#697

docker/Dockerfile

@@ -20,9 +20,9 @@
 FROM centos:7
 MAINTAINER Apache BookKeeper <dev@bookkeeper.apache.org>
 
-ARG BK_VERSION=4.5.0
+ARG BK_VERSION=4.6.0
 ARG DISTRO_NAME=bookkeeper-server-${BK_VERSION}-bin
-ARG GPG_KEY=FD74402C
+ARG GPG_KEY=3ECFCAC4
 
 ENV BOOKIE_PORT=3181
 EXPOSE $BOOKIE_PORT