Providing the environment to buildpacks

[stuartpb]

I think this is an issue? It looked like some of those buildpacks expect config vars.

[stuartpb]

Determining if buildpacks want the environment requires further investigation.

[stuartpb]

https://github.com/motin/dokku-user-env-compile implies it was something provided by a separate plugin, emulating a labs feature at https://devcenter.heroku.com/articles/labs-user-env-compile, which now directs to https://devcenter.heroku.com/articles/buildpack-api - I think it's now just a documented thing a buildpack can do, if it needs it.

Anyway, this can be added with a separate plugin, if/when I've added the hook for adding commands to the build step.

[stuartpb]

Okay, so I've done a little more investigating. user-env-compile is something that can be added with a plugin (although, since the labs feature ended up being deprecated and I'm pretty sure unused now, I'm not sure what that would do. Maybe it would help apps with buildpacks that call scripts that are part of the app and need the config variables defined?)

However, Heroku ended up taking a different approach, as documented on https://devcenter.heroku.com/articles/buildpack-api, where it provides a directory (named in the third argument of the buildpack) of files, similar to what's just been proposed in plushu/plushu-config#4.

Since this passing of arguments to the buildpack's compile happens as part of /build/builder, this needs to be implemented upstream first: progrium/buildstep#81

With the appropriate functionality in place, we can implement this with a build-app-docker-opts hook (basically the same as run-app-docker-opts), and then have plushu/plushu-options hook it to mount the directory from plushu/plushu-options#4 as /env.

[stuartpb]

If/when progrium/buildstep#99 is merged, this can move forward. (Could also use my own fork.)