It's a lame comedy a young Debra Winger as a clueless klutz? but Thank God It's Friday, the only movie ever to costar Donna Summer and Jeff Goldblum, is a fun time capsule. Set in an L.A. disco, with a vague plot about a dance contest, it's really a chance to dig the platform shoes, tight-pantsed butt-wiggling, and brash decadence of the era (a PG movie with casual amyl nitrate poppers?). Director Robert Klane whose résumé as a screenwriter includes Where's Poppa? (1970), European Vacation (1985), and Weekend at Bernie's(1989) has a lurching style that suits the rhythm of the music, but oh, what music. Summer's ''Last Dance'' (which won an Oscar!) and the Commodores' obscure-but-incendiary ''Too Hot to Trot'' remind a new generation that disco did not, as the phrase of the day had it, suck.