Leverage Heroku Release Phase for Auto DevOps

Description

Developers need to automate various tasks during a deploy such as database migrations or publishing static assets. Heroku Release Phase does this via a release entry in Procfile. Perhaps since we're using Herokuish for Auto Build (part of Auto DevOps), we could leverage this. Unfortunately Procfile isn't really a standard outside of Heroku and it's not something we want to encourage generally. And I'm not sure if Buildpacks generate a release definition. (Buildpacks have the ability to generate the equivalent of a Procfile if one isn't defined in the repo. I assume Herokuish is using this at least for the web definition.) But we could define something similar.

Proposal

Links / references