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

templates/bundles/TwigBundle/Exception/error.html.twig

@@ -17,7 +17,7 @@
             Consider yourself lucky. You found some rare error code. <br> You should maybe inform your administrator about it...
         {% endblock %}
     </h3>
-    {% block further_actions %}<p class="help_text">You can try to <a href="javascript:history.back()">Go Back</a> or <a href="{{ path('homepage') }}">Visit the homepage</a>.</p>{% endblock %}
+    {% block further_actions %}<p class="help_text">You can try to <a href="#" onclick="history.back();return false;">Go Back</a> or <a href="{{ path('homepage') }}">Visit the homepage</a>.</p>{% endblock %}
     {% block admin_contact %}<p class="help_text">If this error persists, please contact your
         {% if error_page_admin_email is not empty %}
             <a href="mailto:{{ error_page_admin_email }}">administrator.</a>