In some cases, the cache directory may exist but there may be no app image to use for cache purging. This may be the case if the app was never deployed.
Instead of failing, we can simply use the configured herokuish image for clearing cache, as that image is the base image for anything that would have modified cache.
This change allows operators to specify a DOCKER_BIN environment variable. This will specify a binary to run when executing docker, which is useful in cases where the 'docker' command being run must be modified in a way that would otherwise be invasive to Dokku, but minimalistic if done within a wrapper.
