The amount of alcohol that makes it into breast milk is very low, and the risks to the baby are trivial. Like giving the baby a few drops of alcohol (alcohol content will be something like 1 part per 1000 or less, compare to lager which has 1 part per 20). Worrying about this should be far down people's list of priorities.

Drinking significant amounts while pregnant probably isn't the best idea though, to be sure, since then the alcohol is going straight to baby's bloodstream instead of to the digestive system.