Herokuish app builds currently spin up a temporary container and then commit that container. The new approach uses a Dockerfile to perform those same actions, avoiding the need for the temporary container.
Rather than waiting for the next deploy, take advantage of the retire system to retire these containers immediately.
Note that since the retire process happens out of band, the containers may stick around for up to 5 minutes, after which point they will be removed.
Customization of the wait time to retire - currently 60 seconds - is up for future debate. The containers ultimately haven't been useful for debugging, so keeping them around for longer won't help in most cases, and folks can disable the dokku-retire service (or cron) if that is desired.
This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.
This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.
A builder plugin implements some method for 'building' artifacts from a given source code archive. This refactor moves the majority of the image building code into separate plugins, allowing us to implement other builders, but also simplifying the common pugin codebase.
Also move the pre-build-buildpack hook from the build-env plugin to builder-herokuish.
