There is a route for cyclists but it is signed badly and is very complicated. From the station in Dover ride past Buffers the cafe, third exit at roundabout, then sharp left into small road. Turn right down alleyway, then left, then right and you enter a pedestrian precinct, head southward and down under the main road then turn very sharp right up the ramp and round to the left to exit the underpass. Straight across the next road to the seafront, turn left along the cycle path on the promenade. Cross the main busy road at the set of pedestrian lights, turn left then immediately right into the narrow road fill of parked cars. You arrive at the port and the fun has just begun.
Cross the traffic and follow the poorly painted red line on the ground, this takes you on the right hand side of the traffic flow through the border checks, and instead of entering the huge area for cars you turn sharp right and stop at a barrier where you tell a bloke you're on a bike and need to check in. He gives you a piece of paper that no one ever looks at and lifts the barrier. You park the bike outside the little hut on the right. Here's the important bit, try to look like a cyclist, maybe wear a helmet, and try to push your way to the front by saying you're only a cyclist. Otherwise you get stuck behind drivers with lorries of hazardous goods and they take an age to get through. Hand over piece of paper, receive something you're supposed to hang from your rear view mirror in a car (!) And ride to the lane you're told to. Make sure you're right at the front as they'll load you first and get you off last, so you're not squashed by the lorries.
At Dunkirk make sure you wait until the lorries have gone as the road out is narrow. Ensure you know the route des dunes into the town. All other routes are death.
HTH.