docs: clarify port 1010 bind on Docker for shellypro3em_old

[tomquist]

Summary

Adds a short note to the troubleshooting entry for PermissionError on ports 1010/2220, prompted by #367.

The current bullet recommends "Use Docker", but the published image runs as the non-root astra user, so shellypro3em_old still hits EACCES on port 1010 even with network_mode: host. The note documents the two workarounds that actually work — lowering net.ipv4.ip_unprivileged_port_start on the host or running the container as root — and warns that bridge port-publishing won't help because Marstek discovery uses UDP broadcasts to the subnet address.

Test plan

• Render the Troubleshooting section and confirm the note reads cleanly

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Generated by Claude Code

Summary by CodeRabbit

• Documentation
  • Updated FAQ troubleshooting section with clearer guidance on resolving binding permission errors on privileged ports when running the Docker container. Added explanations for the issue and provided two mitigation approaches.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 9f409f03-396b-48f8-b504-dd8c81c12d4f

📥 Commits

Reviewing files that changed from the base of the PR and between dcf1815 and 98b9522.

📒 Files selected for processing (1)

• README.md

Walkthrough

Added a troubleshooting FAQ note explaining that the Docker image running as a non-root user prevents binding privileged port 1010 with network_mode: host, and documented two mitigation options: adjusting net.ipv4.ip_unprivileged_port_start via sysctl or running the container as root.

Changes

Docker Port Binding Troubleshooting

Layer / File(s)	Summary
FAQ troubleshooting note for Docker privileged port binding README.md	Added FAQ documentation explaining that the non-root container user prevents binding privileged port 1010 with network_mode: host (PermissionError: [Errno 13]), lists sysctl mitigation via /etc/sysctl.d/, root container option, and clarifies that Docker bridge port mapping does not forward UDP broadcast required for discovery.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: clarifying Docker port binding issues for port 1010 on the shellypro3em_old device.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/readme-port-1010-permissions

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