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