Police are seeking a suspect in the robbery of a convenience store inside the Harbor Point apartment complex on the Dorchester waterfront. Officers responded to 17 Harbor Point Blvd. at about 8:09 p.m. Sunday and spoke with a witness who said the thief brandished a gun and told the cashier at the Harbor Point Convenience Store to put money into a bag, according to a statement on the police website. The cashier complied, and the man fled, police said. The robber wore a fake mustache, a black North Face hooded sweatshirt, black jeans, and black-and-white sneakers, police said.