There have been a number of tickets in the past - most recently #7061 - covering the fact that Dokku doesn't clean up disk utilization. The underlying issue is that Docker uses build cache that cannot be cleaned up in a targeted way - and usually doesn't even respect the builder gc settings in a way that makes sense. In fact, the computed disk space does not line up with actual disk utilization, causing it to be a mystery to anyone investigating the underlying problem.
This change introduces a cron-based mechanism that cleans up disk once a day. Cleaning up more often would potentially cause issues during a build if for some reason the prune and an app deploy happened at the same time - its not clear if there is a lock on build cache usage - so cleaning up after hours once a day is a decent tradeoff.
In the future, this setting may be modifiable, but it works well for now.
Closes#7061
