It's worth noting that this is also the approach that e.g. the standard nginx image takes, so if you run nginx in front of your Rails app, you can bring up nginx with a mount of the same volume.

I still prefer to deploy my Ruby apps as part of any image, though. Of course you can combine the two: Build an image whose sole purpose is to package your app and export it as a volume for use by pre-made Rails/Nginx containers.