Perhaps this can go into the build-env plugin as a default env var?
Somewhere like here: https://github.com/progrium/dokku/blob/master/plugins/build-env/pre-build#L10

Why would it need to be in build-env ?

This PR is just adding -e CACHE_DIR=/cache to the docker args of the build container. Figured it'd be cleaner to have all build environment variables in the same place.

Ah makes sense. Seems like it should be in the build-env bit then.

@vincentfretin mind updating your pr to put it in the right place? :)

It doesn't work if I put CACHE_PATH in build-env like CURL_TIMEOUT.
The /app/.env file is read in the main function via buildpack-build which calls buildpack-setup function. The CACHE_PATH is needed before the call to the main function, here: https://github.com/gliderlabs/herokuish/blob/master/include/herokuish.bash#L11

Aw bummer. Welp I guess that settles that. 😄

@michaelshobbs is there a test that might be added for this? I can't think of one, so feel free to merge unless you can.

Doesn't look like there is a good place at this point.