Of note for anyone reading: 1. Since the buildpacks are based on Heroku, if you're having trouble try seeing what it'd take to get your app deployed there. For example with Rails 3.2, if you pre-compile your assets on the production server while deploying an app it will try to instantiate your app and test a database connection; however, Dokku doesn't provide the database configuration until after this step of the deployment so your deploy will fail. This is fixed by either pre-compiling locally or changing a setting in application.rb. In Rails 4 I believe you just have to precompile your assets and check them along with the manifest into your repo. (Ref: https://github.com/progrium/dokku/issues/165, https://github.com/progrium/dokku/issues/202)
2. It's probably easier to just set up a separate database rather than relying on one of the project's plugins. Right now they're just a little too obfuscated and hard to troubleshoot (the dokku-postgresql plugin I used would lose its persistent volume on server reboot).
Have you tried doing a really simple sample app? It may take some coercing to get an existing Heroku app to run (there's some differences between Heroku's buildpacks and Dokku, like in the PHP buildpack). I did write a blog post on the topic of DO-with-Dokku specifically not too long ago, which goes over deploying a "hello world" Node app: http://www.andrewmunsell.com/blog/dokku-tutorial-digital-oce...