A cowardly punk sucker-punched a Brooklyn bodega worker during a scuffle with a rowdy mob outside the Midwood market Monday night — with the shocking attack caught on surveillance video. Police are now looking for the thug who slugged the 36-year-old victim from behind and sent him to the hospital with serious head injuries after workers at the Avenue M store refused to let the troublesome crew inside around 6 p.m., according to dramatic footage of the wild encounter obtained by The Post. A group of about a half-dozen is seen approaching the store, with several going inside — until one of them is stopped at the door and appears to get into a dispute with someone inside.