If you view the Singapore Airlines link below and look at the restrictions for Thailand - point 4.
It says that you can bring aboard domestic flights in Thailand up to 1 litre of liquids purchased duty free. However, the same point then goes on to say:-
Quantities exceeding 1 litre, as well as alcohol purchased before the security checkpoint, must be checked in.
I would take this as you needing to check alcohol in to the hold.
https://www.singaporeair.com/en_UK/gb/travel-info/visas-immigration/customs/international-duty-free-restrictions/