feat: clear out docker builder cache once a day

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

plugins/builder/triggers.go

@@ -99,7 +99,11 @@ func TriggerInstall() error {
 		return fmt.Errorf("Unable to install the builder plugin: %s", err.Error())
 	}
 
-	return nil
+	_, err := common.CallPlugnTrigger(common.PlugnTriggerInput{
+		Trigger: "install-builder-prune",
+	})
+
+	return err
 }
 
 // TriggerPostAppCloneSetup creates new builder files