feat: Docker containerization

Summary

Complete Docker containerization of the GitLab MCP Server with comprehensive
Claude Desktop and Claude Code integration support. This PR enables developers
to run the entire stack (MCP server + Ollama) in Docker Compose with seamless
integration to Claude clients.

What's Included

Docker Implementation

• Multi-stage Dockerfile: golang:1.25.1-alpine builder → distroless/base-debian12 runtime
• Docker Compose: Orchestrates MCP server and Ollama with networking, health checks, and persistent storage
• Environment Configuration: .env file support with Docker-specific overrides
• .dockerignore: Optimized build context
• Build Verification: Image successfully built (61.2MB, arm64/linux)

Automation & Tooling

• Makefile: 40+ targets for development, Docker operations, and CI/CD
• Model Initialization Script: Automated Ollama model pulling with health checks
• Health Checks: Container and service health monitoring

Claude Client Integration

• Claude Desktop: Step-by-step configuration guide for all platforms
• Claude Code: CLI and environment variable integration
• Network Access: Local, remote, and SSH tunneling documentation
• Troubleshooting: Common issues with solutions

Documentation

• docs/DOCKER.md (500+ lines): Comprehensive Docker setup guide
• CLAUDE.md Updates: Docker deployment and Claude Desktop sections
• Configuration Examples: docker-compose.override.yml template