If you’re not a musician or somehow involved in the music industry, the 2004 documentary “Dig!” might seem like “This Is Spinal Tap”, a playful satire poking fun at the excess of musicians who have more ego and drugs than actual talent. But this is no satire. “Dig!” follows two real bands, Portland’s The Dandy Warhols and San Francisco’s The Brain Jonestown Massacre, over seven years from the…