Staying overnight at Dunkerque would be a bit depressing. It is a very bleak spot, and there would probably be lots of lorries around you doing the same thing. If they have refrigeration units they can be quite noisy.
A better option would be to stay at Auchan in Dunkerque - the car park is accessible. It is just off the A16, on the N1 about 3 miles from the port. Website: http://www.auchan.fr/magasin/fichesmagasins/596.asp