Cron containers were never reaped once they exceeded their active deadline, so a hung cron task ran indefinitely instead of being retired after 24 hours as documented. `cron:run` now also accepts a `--ttl-seconds` argument, matching the one `dokku run` already takes.
The docker-local scheduler wrote each app.json cron command verbatim into the dokku user's crontab, where cron's `bash -c` interpreted any shell metacharacters in the command on the host as the dokku user rather than inside the container. The crontab line is now `dokku cron:run <app> <cron-id>`, and the command is resolved from app.json and exec'd inside the container at run time. Commands containing shell operators are also rejected when app.json is validated at deploy time.
`cron:set --global` wrote the property but emitted `unknown flag: --global` because the post-set `scheduler-cron-write` trigger received `--global` as the appName arg, which pflag rejected before reaching the trigger body.
The trigger args now omit appName for global writes, and the `scheduler-k3s` cron-write trigger short-circuits when called without an app since per-app reconciliation requires a real app name. The docker-local trigger already regenerates the global crontab from all apps so global `mailfrom`/`mailto` are picked up without any further changes.
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 functionality enables users to manage a global crontab for the Dokku user that contains all scheduled tasks across all apps. Alternative schedulers can implement cron tasks as desired.
