Add Docker update support via Watchtower integration (#1330)

* Add Docker update support via Watchtower integration

Add web-based Docker container updates using Watchtower HTTP API.
When configured with WATCHTOWER_API_URL and WATCHTOWER_API_TOKEN
environment variables, administrators can trigger container updates
from the Update Manager page.

Features:
- WatchtowerClient service for Watchtower HTTP API communication
- Docker update progress page with animated Docker whale logo
- Real-time step tracking: Trigger, Pull, Stop, Restart, Health Check, Verify
- CSP-compatible progress bar using CSS classes
- Translated UI strings via Stimulus values
- Health endpoint polling to detect container restart
- Watchtower setup documentation for Docker installations
- WatchtowerClient made nullable for non-Docker installations
- Unit tests for WatchtowerClient

* Fixed translation message IDs

* Switch Watchtower docs to maintained nicholas-fedor fork

The original containrrr/watchtower is no longer maintained (last release
Nov 2023). Point users to the drop-in compatible active fork and add an
info note explaining why. No code changes — the HTTP API is identical,
so WatchtowerClient works against either image.

* Fixed exception when github is not reachable

* Only show version string in health endpoint, when user has permissions

* Do not expose watchtower API port in example docker-compose file

* Show if updates, backup restore and backup download are allowed in update manager page

* Report 'not authorized' for version in health endpoint if user lacks permission

---------

Co-authored-by: Jan Böhmer <mail@jan-boehmer.de>

src/EventSubscriber/MaintenanceModeSubscriber.php

@@ -62,8 +62,8 @@ readonly class MaintenanceModeSubscriber implements EventSubscriberInterface
             return;
         }
 
-        //Allow to view the progress page
-        if (preg_match('#^/\w{2}/system/update-manager/progress#', $event->getRequest()->getPathInfo())) {
+        //Allow to view the progress page and health check endpoint
+        if (preg_match('#^/[a-z]{2}(?:_[A-Z]{2})?/system/update-manager/(progress|health)#', $event->getRequest()->getPathInfo())) {
             return;
         }