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.
While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.
The command used to reprocess everything is:
```shell
shfmt -l -bn -ci -i 2 -w .
```
This creates the following commands:
- apps
- apps:create <app>
- apps:destroy <app>
It also:
- makes `delete` an alias for `apps:destroy`
- adds confirmation to `apps:destroy`
- allows a developer to remove apps that have been created via apps:create but have not been deployed
Refs #87
Refs #543
Refs #586Closes#599
Refs #655
Refs #656
Refs #685Closes#757
