Cycling in the time of Corona

I'd call the police/gendarmes in Dunkirk/Calais if I were you. I'd guess that if the ferry company will carry you, you're okay but all you need is one person who decides that you're breaching their interpretation of the rules and you're back on the boat.

Edit. Just checked the French consulate website and it says this:

The following people will be exempt from voluntary quarantine, except if they show symptoms:

• people in transit to another country;
  

So book a hotel or campsite over the border to prove you're heading elsewhere.