add Elixir releases support

[msz]

Elixir 1.9 introduced mix release. This is a huge improvement for Heroku based deployments because the slug can be compiled once and then deployed to different apps with different configs which will be read at runtime.

This PR adds a new option to build the default release on top of the usual compilation process. After the slug is compiled the release can be run by having web: _build/prod/rel/{app_name}/bin/{app_name} start. We at Tallarium already use this setup in all our Elixir Heroku apps.

[tarzan]

...because the slug can be compiled once and then deployed to different apps with different configs which will be read at runtime.

Do you have any examples of how to configure this? Does it also work with automatic deployment through github integration (on heroku)?

[msz]

If your apps are connected in a Heroku pipeline, "Promote to production..." will instantly deploy the staging slug to all production environments without recompilation. This is what I had in mind.

[chrismcg]

I just tried this out, thanks! I ran into a problem with my phoenix app though. The releases are built before the phoenix buildpack generates the asset files so they don't get included. Is there a way to work round this or is something needed in the phoenix buildpack?

[msz]

I can see three ways to go about this:

1. Move hook_post_compile to happen before release. This will allow artifacts from any post-compilation tasks (such as building assets) to be bundled into the release. The asset compilation task would have to be custom written and attached to the hook though.
2. Add an option to generate a release into the Phoenix buildpack as well. It could overwrite the release built by the Elixir buildpack.
3. Create a third buildpack which just generates a release and is run as the last buildpack.

In fact I think option 1. is better behaviour than the current one. I will move the hook.

[chrismcg]

I had a go at option 3 here https://github.com/chrismcg/elixir-mix-release-buildpack. It works a bit better for me as there's a lot in the phoenix buildback I wouldn't want to have to reproduce.

For those just building API or non phoenix apps I think this PR would still be a good thing to have in the Elixir buildpack so it stays a "one stop shop".

[chrismcg]

I've tidied my buildpack up a little, it's at https://github.com/chrismcg/heroku-buildpack-elixir-mix-release now.

[jesseshieh]

@msz this looks like an improvement to me. Do you think you can add a section in the README to describe this option a bit more and explain that it's probably not to be used if you also use the phoenix static buildpack and in that case to use @chrismcg's third buildpack instead?

[msz]

Done!