[DOC IMPROVE] Checkmk2 on the official Docker image: calendar extension and idoitcmk placement

[MichaelOv]

Affected Article

• https://kb.i-doit.com/en/i-doit-add-ons/checkmk2/requirements.html (docs/en/i-doit-add-ons/checkmk2/requirements.md)
• https://kb.i-doit.com/de/i-doit-add-ons/checkmk2/anforderungen.html (docs/de/i-doit-add-ons/checkmk2/anforderungen.md)
• https://kb.i-doit.com/en/i-doit-add-ons/checkmk2/installation.html (and docs/de/...)

Suggested Improvements

The Checkmk2 docs do not cover running on the official i-doit Docker image (verified on the official i-doit app image, version 38, PHP 8.4):

1. calendar PHP extension: the requirements/anforderungen page lists calendar, but the official image does not ship/enable it, so idoitcmk aborts with the Box requirements error requires the extension "calendar". Add a Docker note that it must be enabled on the official image (e.g. via a derived image, see below).
2. idoitcmk placement in a container: the installation page covers the binary + symlink for a normal host, but not the container case: the add-on installer only writes within /var/www, so idoitcmk is not placed in /usr/local/bin automatically (results in idoitcmk: not found). Document setting the configured idoitcmk path (Administration -> Add-ons -> Check_MK) or symlinking, and baking the binary into the image for durable operation (the application tree is otherwise ephemeral).

Optional: Text Proposals

# Base: the official i-doit app image (see the Docker installation guide for the exact image reference)
FROM <official-i-doit-app-image>:38
USER root
RUN docker-php-ext-install calendar
USER www-data