Save DB data persistently in docker-compose sample

Change-Id: I756ecea69ffaf45b44f40c597c26a515ca8c3c73
diff --git a/.gitignore b/.gitignore
index 2bb202b..0e041a9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 docker-compose.yml
 .env
+db-data