Add comprehensive permission issues guide (#580)

- Created PERMISSION_ISSUES_GUIDE.md with detailed solutions for permission problems
- Based on thorough analysis of issues #558, #556, #555, #549, #496, #501, #492, #420
- Includes platform-specific solutions (NixOS, macOS, Windows, Synology)
- Documents critical Docker version requirements (20.x+ recommended)
- Highlights rootless image as recommended solution for permission issues
- Added link to guide in README.md troubleshooting section

Key findings documented:
- Most "Operation not permitted" errors resolved by updating Docker
- Rootless Docker requires using actual Docker UID (e.g., 100999) not 845
- Interrupted chown operations cause inconsistent file ownership
- Rootless images avoid most permission issues entirely

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

README.md

@@ -505,6 +505,19 @@ Consider using rootless images if you:
 
 ## Troubleshooting
 
+### Permission Issues
+
+If you're experiencing permission errors such as:
+- `chown: Operation not permitted`
+- `Permission denied [/factorio/saves]`
+- `Util.cpp:81: Operation not permitted`
+- Files owned by unexpected UIDs (like 100844 instead of 845)
+
+Please refer to our comprehensive [Permission Issues Guide](./PERMISSION_ISSUES_GUIDE.md) for detailed solutions. Common fixes include:
+- **Updating Docker** to version 20.x or newer (this resolves many issues)
+- **Using the rootless image** variants (e.g., `factoriotools/factorio:stable-rootless`)
+- **Setting correct ownership** for your specific Docker configuration
+
 ### My server is listed in the server browser, but nobody can connect
 
 Check the logs. If there is the line `Own address is RIGHT IP:WRONG PORT`, then this could be caused by the Docker proxy. If the the IP and port is correct it's probably a port forwarding or firewall issue instead.