You can buy a pot of growing cress in the UK supermarkets. With smaller dimensions than those listed in #0, though. I would naturally call them a tub of cress or a pot of cress, but I'm finding that punnet of cress googles better in recipes online.
BBC Good Food website has a whole page dedicated to cress (https://www.bbc.co.uk/food/cress) and all the recipes there (who knew there were so many types of cress!) that refer to the container call it a punnet.
Tescos (big supermarket chain) also call their product a punnet of cress, but, unhelpfully, Morrisons and Waitrose just call theirs 'salad cress' and don't name the container...
(#9 - punnets don't need to be cardboard. In fact, my first association is a plastic tub. I normally only associate the word punnet with raspberries and strawberries, but now I evidently need to add 'cress' to that list)