[database] save configuration after DB migration (#3143)

- Make sure that migrated DB contents persisted for next boot
- Make sure that db saved after warm reboot.

Signed-off-by: Ying Xie <[email protected]>

files/build_templates/docker_image_ctl.j2

@@ -89,6 +89,11 @@ function postStartAction()
  if [[ -x /usr/bin/db_migrator.py ]]; then
  # Migrate the DB to the latest schema version if needed
  /usr/bin/db_migrator.py -o migrate
+
+ # Save in memory config_db to config_db.json for 2 reasons:
+ # 1. Persist the DB migration result.
+ # 2. Save in memory DB after warm reboot.
+ /usr/bin/config save -y
  fi
 {%- elif docker_container_name == "swss" %}
  docker exec swss rm -f /ready # remove cruft