David Redmon and Ashley Sabin’s finely detailed documentary “Downeast” immerses itself in Gouldsboro, Maine. It’s a fishing town that was also home to the Stinson Seafood sardine cannery, the last such operation in the United States (there once were as many as 75). The film begins by taking in its last days. The cannery closed in 2010. So the contemplative shots — of fish being caught, fish bulleting out of chutes into vats of thousands of other fish, fish being sliced and sawed at the cannery — achieve the power of visual eulogy. That lasts a few minutes.