Bump ZooKeeper version to 3.8.0 (#21)

diff --git a/Dockerfile b/Dockerfile
index e8151ff..5e1984e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -17,7 +17,7 @@
 
 ARG ACCUMULO_VERSION=2.1.0
 ARG HADOOP_VERSION=3.3.3
-ARG ZOOKEEPER_VERSION=3.7.1
+ARG ZOOKEEPER_VERSION=3.8.0
 ARG HADOOP_USER_NAME=accumulo
 ARG ACCUMULO_FILE=
 ARG HADOOP_FILE=