“The Ringmaster,” screening at the DC Indie Film Festival (March 4-8), is a quirky documentary about onion rings. Not just any onion rings — Worthington, Minn., native Larry Lang’s homemade onion rings. Well, actually, the film isn’t entirely about onion rings; rather it is about Zachary Capp’s efforts to make a documentary about Larry Lang’s onion rings. Capp wants to propel this aging chef’s signature dish from a single restaurant to the world stage. Alas, things don’t quite go as planned. “The Ringmaster,” directed by Molly Dworsky and Dave Newberg, chronicles how Capp, who has an addictive personality — he is a reformed gambling addict — tries to better Lang’s life. However, he manipulates and arguably exploits the chef, for the sake of his self-funded film. (Capp spends boatloads of his inheritance making his documentary over three years until his colleagues tell him, “Enough!”) The humble Larry, who was making his rings in a local saloon, was reluctant to be part of the film. He is also blasé in participating in what might have been …