Ben Affleck goes full-on gangster in his new movie “Live by Night,’’ the trailer for which has finally appeared online. The movie, which Affleck directs and stars in, is based on Dennis Lehane’s Prohibition-era novel about the son of a Boston cop who goes bad. The trailer has the requisite elements of a good gangster flick — great fedoras, vintage cars, a Molotov cocktail, and some poor sap being tossed from a rooftop. This is Affleck’s first directorial effort since “Argo,’’ which won the Academy Award for best picture. “Live by Night,’’ which also stars Zoe Saldana, Sienna Miller, Chris Cooper, Brendan Gleeson, Chris Messina, and Elle Fanning, will be out Jan. 13.