The implementation was very heavily docker-local specific and thus didn't make a ton of sense to keep in the cron plugin.
This also re-introduces changes from #5908 to parallelize the crontab generation (which is also specific to the docker-local scheduler) that were lost during a rebase.
The impetus for this change was implementing a `cron:run` command. The logic for actually running the command isn't scheduler specific - merely invoking `dokku run` with the appropriate arguments - but all the logic around how the commands were retrieved were, so the change was made pre-emptively.
This makes standard use of shellcheck work without needing to provide extra configuration anywhere.
Also remove use of inline 'shellcheck disable' calls that are already defined in the .shellcheckrc and don't need to be set inline.
Previously, we would filter containers and images by name, which could be fail if the app was renamed or no longer was tagged "correctly" due to a rebuild. Now, we filter by label, ensuring an app is completely cleaned up on deletion.
Also:
- Move docker image cleanup from apps plugin to builder plugin: This isn't where it belongs - images should be cleaned up in a builder plugin (or the builders plugin) and not in the apps plugin, which has nothing to do with docker images anyways.
- Remove the code from the scheduler plugin, as that will only do anything if the scheduler is enabled for the app.
- Bypass image removal if there are no images to remove
- Always delete app containers: The herokuish builder creates intermediate build containers - something that should be refactored - and ideally this gets run in that builder plugin, but keeping the logic here ensures we don't have code duplication. This is otherwise safe as it will be a no-op if there are no containers to clean up.
This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.
This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.
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 .
```
