I will check in at Dorsett Hotel HK, via Cebu Pacific Air, can I avail of free shuttle bus service?

Do you refer to Dorsett Mong Kok Hotel? If yes, you should take Airport Express from HKIA to Kowloon Station. Then, take free shuttle bus line K5 to the hotel.
Notably, the free shuttle bus service is only applicable to AE passengers. You need to show AE train tickets (Single Journey, Same Day Return, Round Trip, AE Travel Pass) and Airline ticket / boarding pass before boarding the bus. Have a nice trip!