feat: Docker tiered container management (#31-35)

- DockerManager.php: install Docker CE, engine status, container
  lifecycle (run/stop/start/restart/remove/logs/inspect), image
  management (pull/list/remove), volumes, networks, compose stacks,
  per-user quotas, app catalog with 9 one-click templates
- docker.php API endpoint covering all operations with role-based
  access control (admin/reseller/user isolation)
- DB migration 006: docker_containers, docker_compose_stacks,
  docker_quotas tables
- Admin panel: Docker sidebar link + full management page (containers,
  images, volumes, networks, compose stacks, quota editor)
- Reseller panel: Docker tab with customer container view, quota
  management, and app catalog deployment for customers
- User panel: Docker tab with container dashboard, quota display,
  and self-service app catalog (9 apps: WP, Ghost, Nextcloud, Gitea,
  Matomo, Vaultwarden, Node.js, Flask, Static)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

panel/public/admin/index.php

@@ -106,6 +106,10 @@ $_v = fn($f) => '?v=' . @filemtime(dirname(__DIR__) . $f);
           <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><circle cx="12" cy="12" r="10"/><path d="M2 12h20M12 2a15.3 15.3 0 0 1 4 10 15.3 15.3 0 0 1-4 10 15.3 15.3 0 0 1-4-10 15.3 15.3 0 0 1 4-10z"/></svg>
           WordPress
         </a>
+        <a href="#" class="sidebar-link" data-page="docker">
+          <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><rect x="2" y="8" width="4" height="4" rx="1"/><rect x="8" y="8" width="4" height="4" rx="1"/><rect x="14" y="8" width="4" height="4" rx="1"/><rect x="8" y="2" width="4" height="4" rx="1"/><path d="M2 14c0 4 3 6 10 6s10-2 10-6"/><path d="M20 14c1.5 0 2.5-1 2.5-2.5S21.5 9 20 9h-1"/></svg>
+          Docker
+        </a>
       </div>
 
       <div class="sidebar-section">