Why does Fitzgerald add the detail about the dog-biscuit to the discription of the party at myrtles apartment

This is up to interpretation. The party in the apartment foreshadows Gatsby's great party except it smaller, more visceral, and magnified. The dog biscuit is connected to Myrtle. The dog biscuit is a treat for a dog just as the apartment is a "treat" for the likes of myrtle. In both cases the master controls them. Tom would be Myrtle’s master.