The word “cracker” has several meanings. “Do you serve crackers?”/“We serve everybody” is a popular joke.
“Do you serve crabs here?”/“We serve anyone. Sit down,” “Do you serve fishcakes?,” “Do you serve women at this bar?”/“No, you have to supply your own” and “We don’t serve black people.”/“That’s OK. I don’t eat them” are similar jokes.
Wikipedia: Cracker (term)
Cracker, sometimes cracka or white cracker, is a racial epithet directed towards white people, used especially with regard to poor rural whites in the Southern United States. Although commonly a pejorative, it is also used in a neutral context, particularly in reference to a native of Florida or Georgia (see Florida cracker and Georgia cracker).