You can clone with
HTTPS or Subversion.
We need a heuristic which can tell rails apps apart from any other kind of directory, so we can refuse to attempt to add them to Tokaido if a user tries.
Looking for ../Gemfile is fine to start, but we'll still have a lot of false positives. Something smarter would be better.
I think script/rails is also a good choice.
Why not mirror what Heroku does with their Ruby buildpack?