Use Gerrit 3.0.12 in Dockerfile

Change-Id: I758cac3faa89dbf8984f47d0a609f6be9292f7c5
diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 71e76ba..707e5ae 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:3.0.11
+FROM gerritcodereview/gerrit:3.0.12
 
 ENV GERRIT_BRANCH=stable-3.0