docker: refactor imageRepoName (unit tests pass)
docker: refactor imageRepoName (unit tests pass)

- Note this specialises for github.com repos in code
  but preserves existing tested behaviour.
- Note also that the existing tested behaviour matches
  all known real life usage as well.

samsalisbury/sous,opentable/sous,samsalisbury/sous,samsalisbury/sous,opentable/sous,samsalisbury/sous,opentable/sous,opentable/sous,samsalisbury/sous,opentable/sous