[RELEASE] Update Dockerfile to 4.9.0






Reviewers: Jia Zhai <zhaijia@apache.org>

This closes #1929 from sijie/update_docker_file

(cherry picked from commit 1da10f8cfaa7ad2bd132f7136291e20ce2d97d5c)
Signed-off-by: Sijie Guo <sijie@apache.org>
diff --git a/docker/Dockerfile b/docker/Dockerfile
index e63ab77..d8639d8 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -20,9 +20,9 @@
 FROM centos:7
 MAINTAINER Apache BookKeeper <dev@bookkeeper.apache.org>
 
-ARG BK_VERSION=4.8.1
+ARG BK_VERSION=4.9.0
 ARG DISTRO_NAME=bookkeeper-server-${BK_VERSION}-bin
-ARG GPG_KEY=A615D22C
+ARG GPG_KEY=FD74402C
 
 ENV BOOKIE_PORT=3181
 EXPOSE $BOOKIE_PORT