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

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";
+    }
+}

docker/etc/runit/runsvdir/default/dokku-restore/finish

@@ -10,6 +10,10 @@ if [[ "$1" -ne 0 ]]; then
 fi
 
 echo "Done restoring dokku apps."
-exec sleep infinity
 
-exit 0
+while ! (echo > /dev/tcp/127.0.0.1/22) >/dev/null 2>&1; do sleep 1; done
+while ! (echo > /dev/tcp/127.0.0.1/80) >/dev/null 2>&1; do sleep 1; done
+touch /var/run/dokku/ready
+echo "Dokku is ready."
+
+exec sleep infinity