docs: Write self-hosting guide

[ggfevans]

Summary

Document self-hosting Rackula with Docker.

Migrated from: RackulaLives/www#19

Content Outline

Docker Deployment

• Quick start with Docker
• docker-compose.yml example
• Environment variables

Reverse Proxy

• Nginx configuration
• Traefik configuration
• Caddy configuration

Updates

• Updating to new versions
• Backup procedures

Acceptance Criteria

• Working Docker examples
• Reverse proxy configs for common setups
• Troubleshooting section