http://cruisesouthampton.com/markers/dover-priory-rail-station
Scroll down - read paragraph 2 & 3 in conjunction wivva map.
Having said which...... the suggested route looks like you'll be mixing it with artics in a not very pleasant way.
There does appear to be a cycle path (check out the satellite view on Google maps) which runs parallel to the A20 to the south of it, with a controlled crossing of the A20 and a further cycle path to the north of the A20 which looks like it takes you straight into the ferry terminal.
Edit : I've sent a tcx file to your email. Worth taking a look here
http://www.doverport.co.uk/ferry/ as well.
Apparently, once you enter the port, there's a red route line painted onto the road surface to guide cyclists into the terminal.