It seems like there is a competition for everything today, from beard-growing to air guitar to chicken-wing-eating. Meet the latest addition to the world of competition: grocery bagging. And, like air guitar, Scrabble, and spelling bees, somebody has made a documentary about the phenomenon. "Ready, Set, Bag!" premieres in San Francisco tomorrow night at the Roxie (7pm), followed by a Q&A with the film's directors. The documentary follows eight contestants who train for the National Grocery Bagging Competition in Las Vegas by relentlessly packing grocery bags as efficiently as possible as fast as possible.