Andrew,
Most weeks I drive past Dunkerque on my way home from work in Rotterdam, so I normally do the family food shopping in the Auchan Hypermarket just off the A16
Unlike Calais, the Dunkerque Auchan hypermarket stocks regional food and produce (Nord) - so you will see Flanders beer on offer and a great selection of locally produced foods.
Wine - the Auchan own-label varietal wines (Merlot, Cab. Sav., Sav. Blanc, Chardonnay, etc) are outstandingly good for Euro 2 - produced in France using New World wine making skills these are very much to UK / Dutch tastes. The bottles have a plain white label with a large image of a vine leaf, the wine variety and the Auchan own-label cardboard neck decorations.
Auchan Hypermarket - about 400 metres from junction 25 off the A16 - direction Grande Synthe.
Watch out for the speed cameras on the A16.
Robert