Save DB dump after warm/fast reboot (#8803)

As a part of warmboot, redis database is dumped:
https://github.com/Azure/sonic-utilities/blob/c97fe546e5a725b9049047293a4fede48fde5223/scripts/fast-reboot#L269
However, this dump file is deleted, after it is loaded back into db post reboot.
The DB dump can be useful for debugging purpose, hence taking a backup of it can be useful.
Instead of deleting the dump, rename and keep the dump.

files/build_templates/docker_image_ctl.j2

@@ -143,7 +143,8 @@ function postStartAction()
           sleep 1;
         done
         if [[ ("$BOOT_TYPE" == "warm" || "$BOOT_TYPE" == "fastfast") && -f $WARM_DIR/dump.rdb ]]; then
-            rm -f $WARM_DIR/dump.rdb
+            # retain the dump file from last boot for debugging purposes
+            mv $WARM_DIR/dump.rdb $WARM_DIR/dump.rdb.old
         else
             # If there is a config_db.json dump file, load it.
             if [ -r /etc/sonic/config_db$DEV.json ]; then