ATLANTA — Gangs at some Georgia prisons have been using cellphones to traffic drugs, smuggle in contraband, steal identities and, in at least one case, to arrange a violent attack on an inmate suspected of snitching, according to federal indictments unsealed Thursday. Officials say a dozen people were involved in two prison-based crime rings, and one of those inmates is Ruben Ruiz, aka “Flaco” and “Scrappy,” 34, of Gainesville. The inmates used prison employees to smuggle cellphones and other contraband into the prisons, two indictments say.