Hardcode stack name

The newer heroku buildpackes (for example ruby/rails) use the STACK
variable for caching and cache busting (due to the cedar-14 stack).
Without a STACK envvar the caching system is broken and results in the
whole set of gems being reinstalled

For more info please visit: https://github.com/dokku-alt/progrium-buildstep-dockerfiles/pull/2