I'm not maintaining this plugin. I believe there is an active fork here: https://github.com/progrium/dokku/wiki/Plugins
dokku-user-env-compile is to dokku what user-env-compile is to heroku. It essentially makes a applications ENV file available to the build container. This allows the build process to to run in an environment that more closely resembles the runtime environment.
cd /var/lib/dokku/plugins git clone https://github.com/musicglue/dokku-user-env-compile.git user-env-compile dokku plugins-install