XWiki: increase installer execution timeout and add retries

The task 'XWIKI | Execute installer page' now uses:
- timeout: 300 (allow up to 5 min per request)
- retries: 20
- delay: 15
- until: condition

This prevents early failures during the first Distribution Wizard bootstrap when hundreds of extensions are still being installed.

Context: https://chatgpt.com/share/68ca0f18-2124-800f-a70d-df1811966107

roles/web-app-xwiki/templates/Dockerfile.j2

@@ -1 +1,5 @@
 FROM {{ XWIKI_IMAGE }}:{{ XWIKI_VERSION }}
+RUN cat >> /usr/local/tomcat/webapps/ROOT/WEB-INF/xwiki.cfg <<EOF
+xwiki.superadmin=1
+xwiki.superadminpassword={{ XWIKI_SUPERADMIN_PASSWORD }}
+EOF