Nearly 500 workers at Caritas Carney Hospital in the Boston neighborhood of Dorchester will join the Service Employees International Union, making it the second Caritas Christi Health Care hospital to unionize this spring. In April, more than 800 employees at St. Elizabeth's Medical Center in Brighton voted to affiliate with the same labor union, Local 1199 of SEIU United Healthcare Workers East. The vote at Carney was 299 to 78, with about 79% of the 377 workers who cast ballots opting to join the union.