Use custom image for mysql

Change-Id: Ia0552b91eed2ff4b792aa13a9eefc3cf336871d6
diff --git a/docker/swarm/stack/gerrit.yml b/docker/swarm/stack/gerrit.yml
index 26ffe7f..788d97e 100644
--- a/docker/swarm/stack/gerrit.yml
+++ b/docker/swarm/stack/gerrit.yml
@@ -1,7 +1,7 @@
 parameters:
   _param:
     docker_image_gerrit: tcpcloud/gerrit:2.13.6
-    docker_image_mysql: mysql:5.6
+    docker_image_mysql: tcpcloud/mysql:5.6
     gerrit_ldap_server: ""
     gerrit_ldap_bind_user: ""
     gerrit_ldap_bind_password: ""