Hi @Jack_Martin, welcome to the CodeCombat Discourse! 
I tried setting up CoCo with docker a few days ago and unfortunately it didn’t work for me either. I would recommend trying this method instead (it worked for me):
The “Simplified” Guide to Setting Up Dev Environment. (Mac)
However localhost:3000 didn’t work on Chrome or Safari for me. Apparently it’s a known error which has recently cropped up and the team will try and fix it soon.
Using the guide should get CoCo on your computer anyway, and when the issue is resolved you’ll be able to start doing whatever you plan to do with it. Also you could try Firefox which works for some people (I haven’t tried it yet).
If you have anymore problems I would recommend going to the development chat where they know lots about that kind of stuff: https://coco-slack-invite.herokuapp.com/
Danny