There is no road to the dock where you will be tendered ashore. Your options are walking up 600+ donkey poo-covered steps or taking the cable car. Each step is 4-5 strides deep so it takes a lot of effort. Please DON'T ride the poor abused donkeys!
Until last August there was also the option of taking a water taxi from the tender dock straight to Amoudi Bay below Oia. The ticket price, €20, included a shuttle van up into Oia and a bus ticket back to Fira. From Fira you have to walk down the steps or take the cable car. This water taxi was discontinued because of unspecified "legal issues" and so far there's been no indication that it has been re-instated, but if it has that would be the best way to avoid the long lines for the cable car ride. The cable car only takes 36 passengers at a time, and if more than one ship is in port that day it can take a very long time to get up the cliff. I've read complaints of as much as a 2-hour wait. Good luck, I hope it all works out!
Cruise ship calendar: http://www.santorini.net/information/cruise-ship-calendar