Scheduled cron task output previously reached only the `dokku` user's cron mail, and could not be redirected because `app.json` rejects bare shell operators in a cron `command`. Setting `vector-cron-sink` on an app or globally routes that output to a dedicated sink instead, on both the `docker-local` and `k3s` schedulers, which keeps log destinations under operator control rather than in a deployed repository. Cron events carry `dokku_app` and `dokku_cron_id` fields so a sink can give each task its own destination. This also fixes a `k3s` bug where configuring a global `vector-sink` silently removed the vector prometheus exporter sink.
Dokku uses a few external images for various bits of functionality, but these must be manually updated every so often due to being referenced in code.
By moving the references to Dockerfiles, we can take advantage of dependabot to issue automatic PRs when they are out of date.
Using compose instead of manual docker calls allows folks to customize the vector container by using a custom compose.yml template file. This opens us up to more customizations while aligning container management with how we do other external containers (such as for the proxy plugins).
Refs #5784
Not all properties can be set globally, and the "appName" in the global case is currently defaulted to `--global`. This is okay because domain label names cannot start with hyphens, and all app names must be valid domain label names.
Moving verification of input to the earliest place is the best way to decrease the need to validate this further downstream.
Additionally, some of the existing checks were.... lacking :)
