Use Gerrit 3.1.7 in Dockerfile

Change-Id: Ie2e152fc32eabfd4ca59949d4beea0ed4c327bcb
diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index bfce2c4..f7204b4 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:3.1.5
+FROM gerritcodereview/gerrit:3.1.7
 
 ENV GERRIT_BRANCH=stable-3.1