fix: ensure the dokku-retire timer is properly installed

Closes #3615

tests/unit/30_scheduler-docker-local.bats

@@ -0,0 +1,18 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+}
+
+teardown() {
+  global_teardown
+}
+
+@test "(scheduler-docker-local) timer installed" {
+  run /bin/bash -c "systemctl list-timers | grep dokku-retire"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}