The bare `cron` dependency caused `apt` to remove `dokku` whenever a user installed an alternative cron daemon such as `systemd-cron`. Listing the virtual `cron-daemon` package as an alternative keeps the default install behaviour (fresh installs still pull in `cron`) while letting any provider of `cron-daemon` satisfy the dependency, so swapping cron daemons no longer takes `dokku` with it.
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