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.}

I would say “light” as it is the most idiomatic way to say “low-cal” in French when referring to soda and other beverages. “Allégé” is sometimes used as well, e.g. for cream.

But I must say I don’t recall ever seeing such a beer anywhere in France. French people tend to classify beer mostly by colour, even if it’s a bad proxy for taste, and possibly by their alcohol content. That might be why you failed to find any common phrase for this.

What you will find instead is low-alcohol beer, which is inherently low-cal, but it’s not called that. It’s called “sans alcool” or “0%” instead (there is a subtle difference, “0,0%” only appeared recently with better filtering techniques, older “no alcohol” beers do in fact contain a little alcohol).