feat: simplify Docker config file selection with convention over configuration

Implement automatic detection of custom_config.json for simpler Docker usage.
No environment variables or command-line arguments needed for most users.

Changes:
- entrypoint.sh: Add smart config detection (custom_config.json > CLI arg > default_config.json)
- docker-compose.yml: Add configs volume mount for editing without rebuilds
- docs/DOCKER.md: Update documentation with simplified workflow
- .gitignore: Add custom_config.json to prevent accidental commits

Usage:
  cp configs/default_config.json configs/custom_config.json
  nano configs/custom_config.json
  docker-compose up  # Automatically uses custom_config.json

Simplifies user experience by following convention over configuration principle.

docker-compose.yml

@@ -7,6 +7,7 @@ services:
     volumes:
       - ./data:/app/data
       - ./logs:/app/logs
+      - ./configs:/app/configs
     environment:
       # AI Model API Configuration
       - OPENAI_API_BASE=${OPENAI_API_BASE}