fix(controller): do not wrap buildpack processes with bash

slugrunner's entrypoint is `/runner/init`, which [execs the process](https://github.com/deis/deis/blob/master/builder/image/slugrunner/runner/init#L61-L66) with `bash -c "<process>"`.

Also fixes an issue with the new builder changes. The `dockerfile` attribute from buildpack apps was set as `False` rather than an empty string, causing erratic behaviour with this PR.