Needs an option to enable the Heroku release phase

[danthedaniel]

Heroku requires the release process be specified in order to use the new container as the source of the release command:

https://devcenter.heroku.com/articles/container-registry-and-runtime#release-phase

Ex:

$ heroku container:release web release
Releasing images web,release to your-app-name... done
Running release command...
Migrating database.

Without this I was having trouble as the release script referenced the last Github-originated build and not the most recent build pushed to the Heroku container registry.