This "quest" must be completed three times before Rocknot will move to the large kegs in the room.
You can buy six Dark Iron Ale Mugs from Plugger Spazzring, or pick them up from the table near Spazzring. (Warning: The latter option will end up turning Spazzring hostile to your character.)
Once you've given Rocknot enough booze, he stands up and tries to get even more brew from the kegs. Unfortunately for him, the first two are empty and the third keg bursts open explosive force. The stopper from the third keg knocks open the door keeping you from progressing further in the dungeon. Phalanx activates due to the property damage, providing your character with another dungeon boss.
A rogue can Pickpocket the key to the door from Spazzring instead, but this causes the bar to become hostile.
There is another option, but it's very, very unlikely that you will have the following character in your LFG dungeon party: someone who completed the quest The Love Potion prior to its removal in Cataclysm. That character can speak to Mistress Nagmara and she will, ahem, escort Rocknot out of the bar, unlocking the door in the process. (I prefer doing this when on my main character - no muss, no fuss.)