Add documentation for ARMBIAN_DOCKER_AUTO_PULL build switch

[igorpecovnik]

Summary

• Add documentation for the new ARMBIAN_DOCKER_AUTO_PULL build switch
• Document automatic Docker image pulling feature via system cronjob
• Include usage examples and system file locations

Changes

• Added comprehensive documentation for ARMBIAN_DOCKER_AUTO_PULL in the "Host environment" section
• Documented all three options: yes, no, and undefined (default)
• Included practical examples showing how to enable and disable the feature
• Added technical details about system files created when the feature is enabled

Context

This documentation supports the feature being developed in armbian/build#9265, which adds automatic Docker image pulling and cleanup functionality.

Test plan

• Documentation follows existing format and style
• Examples are clear and accurate
• Placed in logical section (Host environment, near other Docker options)

Documentation website preview will be available shortly:

 Open WWW preview 

[coderabbitai]

Walkthrough

This pull request adds documentation for a new build switch ARMBIAN_DOCKER_AUTO_PULL in the Developer Guide. The change introduces a string parameter with three possible values (yes, no, and undefined as default) that enables automatic background updates of Docker build images via a scheduled cronjob every 12 hours, including cleanup logic to retain the 2 most recent images per tag. The documentation includes configuration examples, usage notes, and details about generated files and cron scheduling.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Suggested labels

Needs review, size/small

Suggested reviewers

• EvilOlaf

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and accurately summarizes the main change: adding documentation for the ARMBIAN_DOCKER_AUTO_PULL build switch.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The PR description clearly describes the documentation changes, including the new ARMBIAN_DOCKER_AUTO_PULL build switch, its options, examples, and context.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch docs/docker-auto-pull-switch

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