Save DB data persistently in docker-compose sample

Change-Id: I756ecea69ffaf45b44f40c597c26a515ca8c3c73
diff --git a/docker-compose.sample.yml b/docker-compose.sample.yml
index 6f8dc16..285d81e 100644
--- a/docker-compose.sample.yml
+++ b/docker-compose.sample.yml
@@ -10,6 +10,8 @@
       - db
     networks:
       - backend
+    volumes:
+      - ./db-data:/var/lib/mysql
     restart: unless-stopped
   db:
     image: mariadb