feat: add docker healthcheck to dokku container

The official dokku/dokku image gains a HEALTHCHECK directive backed by a loopback-only HTTP endpoint at `127.0.0.1:18080/_dokku/health`. The endpoint reports 200 once first-boot bootstrap finishes, sshd and nginx are accepting connections, and `dokku ps:restore` completes; otherwise it returns 503. Changes are scoped to the Docker overlay and Dockerfile so debian-package installs are unaffected.

docker/etc/my_init.d/10_dokku_init

@@ -35,6 +35,9 @@ main() {
 
   support-userns
 
+  rm -f /var/run/dokku/ready
+  mkdir -p /var/run/dokku
+
   if [[ ! -d /mnt/dokku ]]; then
     log-info "Creating missing /mnt/dokku"
     mkdir -p /mnt/dokku