“Detroit” is real-life tragedy, a bloody slice of America’s racial history. The setting is the Detroit riot of July 1967, one of the largest urban uprisings of the 20th century. Director Kathryn Bigelow and writer Mark Boal — collaborators on “The Hurt Locker” and “Zero Dark Thirty” — follow their insightful stories of U.S. police actions in Iraq and Afghanistan with a sharp study of conflict on the home front, re-creating a single horrifying event that illuminates our larger, ongoing crisis.