fix(controller): revert to `start proctype` for buildpack images

The slugrunner image virtually requires that we use `start web` and `start worker` for process types. This reverts back to the old behaviour only for Heroku-compatible apps such that we are backwards compatible with the invoked commands for those apps, while Dockerfile and Docker Image deployments still take advantage of the new features available.

Also fixes an issue with the recent builder changes. The `dockerfile` attribute from buildpack apps was set as `False` rather than an empty string, causing erratic behaviour with this PR.