Use Gerrit 3.1.8 in Dockerfile

Change-Id: I74f285ae0b3a9c21e7344321c7e086ed55c87903
diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index f7204b4..4a1dfbb 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:3.1.7
+FROM gerritcodereview/gerrit:3.1.8
 
 ENV GERRIT_BRANCH=stable-3.1