docs: improve custom sandbox guide with detailed requirements and examples

[h55nick]

This PR improves the custom sandbox guide documentation by:

1. Adding a clear "Important Requirements" section highlighting:

   • Debian-based image requirement
   • Root user requirement
   • System-wide package availability

2. Providing a complete example Dockerfile with multiple packages

3. Adding two methods for building and running:

   • Direct Docker build
   • Docker Compose setup

4. Including detailed sections on:

   • Configuration options
   • Common issues and troubleshooting
   • Testing procedures
   • Security considerations

This addresses issue #5266 by making the custom sandbox setup process more clear and comprehensive.

Changes include:

• Added clear requirements section
• Improved example Dockerfile
• Added docker-compose.yml example
• Added troubleshooting guide
• Added testing instructions
• Added security considerations