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

docker/etc/cron.d/docker-builder-prune

@@ -0,0 +1,4 @@
+PATH=/usr/local/bin:/usr/bin:/bin
+SHELL=/bin/bash
+
+5 4 * * * root /usr/local/bin/docker builder prune >>/var/log/dokku/builder-prune.log 2>&1

plugins/builder/install-builder-prune

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+trigger-builder-install-prune() {
+  declare desc="builder install plugin trigger"
+  declare trigger="install"
+
+  if [[ $(systemctl 2>/dev/null) =~ -\.mount ]]; then
+    cat <<EOF >/etc/systemd/system/docker-builder-prune.service
+[Unit]
+Description=Docker builder prune service
+Requires=docker.service
+After=docker.service
+
+[Service]
+Type=oneshot
+User=$DOKKU_SYSTEM_USER
+ExecStart=$DOCKER_BIN builder prune
+
+[Install]
+WantedBy=docker.service
+EOF
+
+    cat <<EOF >/etc/systemd/system/docker-builder-prune.timer
+[Unit]
+Description=Run docker-builder-prune.service every day at 4:05am
+
+[Timer]
+OnCalendar=Mon..Sun 04:05
+Persistent=true
+
+[Install]
+WantedBy=timers.target
+EOF
+    if command -v systemctl &>/dev/null; then
+      systemctl --quiet reenable docker-builder-prune
+      systemctl --quiet enable docker-builder-prune.timer
+      systemctl --quiet start docker-builder-prune.timer
+    fi
+  else
+    cat <<EOF >/etc/cron.d/docker-builder-prune
+PATH=/usr/local/bin:/usr/bin:/bin
+SHELL=/bin/bash
+
+5 4 * * * $DOKKU_SYSTEM_USER $DOCKER_BIN builder prune >>/var/log/dokku/builder-prune.log 2>&1
+EOF
+  fi
+}
+
+trigger-builder-install-prune "$@"

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