fix(controller): use /runner/init entrypoint only on procfile

When a dockerfile and a Procfile are present, the default entrypoint
should be /bin/bash instead of /runner/init, which is the default
entrypoint for Procfile-based applications.
ref(controller): use /bin/sh for better portability

This allows us to run admin commands on micro-images such as busybox.
Merge pull request #2642 from bacongobbler/fix-entrypoint

fix(controller): dockerfile entrypoint should use /bin/sh