Not really idiomatic but I believe bière light is much more used than the other suggestions, at least in France.
If avoiding the anglicism is needed, I would use Bière légère* like Coors is doing, all remaining choices (allégée, basse calories, faible en calories) being more or less unattractive.
I wouldn't use bière sans alcool either, given the fact well known light beers like Bud Light and similar are not alcool-less.
* As the various comments and other answers clearly show, "Light beer" is already ambiguous is English. Retaining "Light" in Bière light or using the controversial légère in Bière légère to translate "light" keeps this ambiguity which is what a good translation should do, imho.