dokku-alt/progrium-buildstep-dockerfiles — pull request #2 by yabawock (2014-09-21 18:05:35+00:00)
Title: Provide stack name 

The newer heroku ruby buildpackes (after heroku/heroku-buildpack-ruby@3e01277) use the STACK
variable for caching and cache busting. Without the STACK envvar the bundler cache will be emptied on each deploy and cause all gems to be redownloaded.