It’s never a good thing when the postscript to a film based on a true story is more interesting than the actual movie itself. This is unfortunately the case with Patrick Gilles’ “I’m Charlie Walker,” a biopic about an enterprising Black trucker who won a lucrative contract to help clean up the largest oil spill in San Francisco Bay history in 1971 when two Standard Oil tankers collided. As interview footage of the real Charlie Walker plays with text cards just before the credits roll, it’s clear that this film would have been better as a documentary. “I’m Charlie Walker” has all the makings of…