Save DB dump after warm/fast reboot (#8913)

Back porting the master branch change - #8803

Save the redis DB dump after warm reboot.

files/build_templates/docker_image_ctl.j2

@@ -50,7 +50,8 @@ 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 dump file, load it
         if [ -r /etc/sonic/config_db.json ]; then