Residents were displaced after a Dorchester house caught fire Sunday night. The one-alarm blaze ignited the second-floor ceiling and attic of the 2½-story house at 54 Dracut St., Boston Fire Department spokesman Steve MacDonald said. Firefighters arrived at about 10:40 p.m.; most of the fire had been put out within an hour, he said. No injuries were reported. MacDonald said he did not know how many people were displaced by the fire. He said investigators will try to determine the cause and how much damage the fire caused.