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/nginx/conf.d/zz-dokku-health.conf

@@ -0,0 +1,16 @@
+# Internal health endpoint for the official dokku/dokku Docker image.
+# Bound to loopback so it is reachable only from inside the container
+# and never conflicts with user app vhosts on 80/443.
+server {
+    listen      127.0.0.1:18080;
+    server_name _;
+    access_log  off;
+    default_type text/plain;
+
+    location = /_dokku/health {
+        if (-f /var/run/dokku/ready) {
+            return 200 "ok\n";
+        }
+        return 503 "not ready\n";
+    }
+}