Use Gerrit 2.16.18 in Dockerfile

Change-Id: I1d2e0b308dcdedd538a8bc66ab83ebfb9dfa3b56

src/test/docker/gerrit/Dockerfile

diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index ec38c49..2d2aed0 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:2.16.17
+FROM gerritcodereview/gerrit:2.16.18
 
 ENV GERRIT_BRANCH=stable-2.16