Use Gerrit 2.15.18 in Dockerfile

Change-Id: I28658ae1a9aaf11fd419fbbaee3b51cf60d5034a

src/test/docker/gerrit/Dockerfile

diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 2580e96..fb4ab9c 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:2.15.17
+FROM gerritcodereview/gerrit:2.15.18
 
 ENV GERRIT_BRANCH=stable-2.15