docs: Docker installation guide using official synetics images

[MartinHartkopf]

Summary

• Adds a new German page docs/de/installation/docker-installation/compose.md describing how to deploy i-doit Pro with the official synetics container images (registry.on.ops.docupike.net/i-doit/app:38, db:38) via Docker Compose, including an optional nginx-based TLS reverse proxy.
• Extends the existing Docker-installation index to present the two installation paths side by side: the new compose-based deployment with official images, and the existing custom build on Debian/Ubuntu.
• Registers the new page in the German navigation (config/de/mkdocs.yml).

Why

i-doit now ships official, hardened container images from the synetics tech-ops registry. They are simpler to deploy than the custom Apache/PHP build the existing Docker articles describe, but the entrypoint has a few sharp edges that aren't obvious from the upstream compose.yaml snippet alone:

• Missing IDOIT_VERSION / IDOIT_SYSTEM_SETTINGS in the .env crashes the app container immediately (set -u in the entrypoint).
• A tmpfs mount on /var/www/html/temp that appears in some templates collides with the add-on installer's rmdir of that directory.
• The image initialises i-doit in cloud mode; getting Pro mode requires an explicit idoit:feature-manager --unset-cloud / --noncloudable --enable all step plus toggling $g_is_cloud in config.inc.php.
• IDOIT_APP_URL is only persisted on the very first container start, so a wrong value (e.g. http://localhost:8080) needs the idoit:set-env-var console command to fix.
• Behind a TLS reverse proxy, Apache's directory-slash redirect emits absolute http://...:8080/ URLs that the proxy has to rewrite (proxy_redirect).

All of these are documented in dedicated troubleshooting sections so users don't have to rediscover them.

Test plan

Verified end-to-end on a clean Debian 13 (Trixie) VM, two independent runs from a fresh snapshot:

• docker compose pull && docker compose up -d brings the stack to healthy within ~30 s.
• curl http(s)://.../health returns {"status":"ready","version":"38"}.
• HTTP request is redirected (301) to HTTPS by nginx; login page served over HTTPS.
• Login + logout flow over HTTPS works (no stray localhost:8080 redirect).
• Admin-Center link works over HTTPS (proxy_redirect rewrite verified).
• Pro-mode activation steps complete successfully; cloud indicators disappear.
• Add-on install from the Subscription Center succeeds (no rmdir: Device or resource busy).
• docker compose restart and docker compose down (without -v) followed by up -d preserve app and db volumes; stack returns to healthy.

Follow-up

An English translation of compose.md and the corresponding config/en/mkdocs.yml update will follow in a separate pull request.

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