I am using v6 trial version and cannot find where to add the docker repository.  There is a docker icon under settings and can turn it on there but I cannot find how to add repository or set one up for that matter.

After turning on docker in the settings (and setting up an image file), there should be a new tab appear (Docker) and within it there is a section called Repositories.

 

Or alternatively, install the Community Applications plugin  http://lime-technology.com/forum/index.php?topic=40262.0 to eliminate the need for repositories altogether.