[DOC IMPROVE] Docker installation: document persistence of add-ons and custom language files

[MichaelOv]

Affected Article

• https://kb.i-doit.com/en/installation/docker-installation/compose.html (docs/en/installation/docker-installation/compose.md)
• https://kb.i-doit.com/de/installation/docker-installation/compose.html (docs/de/installation/docker-installation/compose.md)
• Cross-reference: administration/multilingual-support.md / mehrsprachigkeit-und-uebersetzungen.md

Suggested Improvements

The compose guide documents that the app and db volumes persist and how to back them up, but not which parts of the application tree are ephemeral, nor how to keep data i-doit writes into the webroot. On the official i-doit app image (version 38, PHP 8.4) these are lost on container recreate. Add a "Persistence" section covering:

1. Add-ons: installing via the Subscription Center / UI unpacks into the ephemeral application tree (src/classes/modules/…) and is lost on recreate. Persistent path: place the add-on ZIP into /data/data/addons-to-install/; the entrypoint installs it and moves it to /data/data/addons/ (symlink). Not documented today.
2. Custom translations / renamings: i-doit writes src/lang/<abbr>_custom.inc.php (see administration/multilingual-support.md) into the application tree, so it is lost on recreate. Persist via a derived image (COPY) or a bind-mount (must be writable by the web user, uid 33). A behavior fix to persist these automatically is planned.

Optional: Text Proposals

# Derived image: persist custom translations
COPY de_custom.inc.php /var/www/html/src/lang/de_custom.inc.php
# Add-ons: place the ZIP into the persistent /data/data/addons-to-install/ before start