Update BookKeeper version in Docker file for 4.8.1 release

Descriptions of the changes in this PR:
Change version from 4.8.0 in 4.8.1 on Dockerfile in branch-4.8


### Motivation

Distribute new Docker images for 4.8.1 release





Reviewers: Sijie Guo <sijie@apache.org>

This closes #1851 from eolivelli/fix/docker-4.8.1
diff --git a/docker/Dockerfile b/docker/Dockerfile
index deb00ea..e63ab77 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -20,7 +20,7 @@
 FROM centos:7
 MAINTAINER Apache BookKeeper <dev@bookkeeper.apache.org>
 
-ARG BK_VERSION=4.8.0
+ARG BK_VERSION=4.8.1
 ARG DISTRO_NAME=bookkeeper-server-${BK_VERSION}-bin
 ARG GPG_KEY=A615D22C