[fast-reboot] Fix regression: set FAST_REBOOT state_db flag to support fast-reboot from older images (#16733) (#16753)

files/build_templates/docker_image_ctl.j2

@@ -249,6 +249,12 @@ function postStartAction()
                     $SONIC_CFGGEN -j /etc/sonic/config_db$DEV.json --write-to-db
                 fi
             fi
+
+            if [[ "$BOOT_TYPE" == "fast" ]]; then
+                # this is the case when base OS version does not support fast-reboot with reconciliation logic (dump.rdb is absent)
+                # In this case, we need to set the flag to indicate fast-reboot is in progress. Set the key to expire in 3 minutes
+                $SONIC_DB_CLI STATE_DB SET "FAST_REBOOT|system" "1" "EX" "180"
+            fi
         fi
 
         if [ -e /tmp/pending_config_migration ] || [ -e /tmp/pending_config_initialization ]; then