Add Docker Compose support for development with separate containers

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This PR implements comprehensive Docker Compose support for development, providing separate containers for each functionality as requested in the issue.

🐳 Services Implemented

• webapp-mysql: Rails application with MySQL database (accessible at http://localhost:3000)
• webapp-postgresql: Rails application with PostgreSQL database (accessible at http://localhost:3001)
• db-mysql: MySQL 8.0 database server
• db-postgresql: PostgreSQL 13 database server
• redis: Redis cache server for production-like development

📁 Files Added

• Dockerfile: Optimized Rails container with all necessary dependencies
• docker-compose.yml: Complete service orchestration with health checks
• entrypoint.sh: Smart database initialization and wait logic
• config/database.yml.docker: Environment-based database configuration
• docker-dev.sh: Developer-friendly command interface
• test-docker-setup.sh: Comprehensive validation suite
• demo-docker-setup.sh: Setup demonstration script
• .dockerignore: Optimized build context
• .env.example: Configuration template with all available options

🚀 Usage

Quick Start

# Start with MySQL
./docker-dev.sh up-mysql

# Start with PostgreSQL
./docker-dev.sh up-postgres

Development Commands

./docker-dev.sh shell-mysql      # Open development shell
./docker-dev.sh console-postgres # Rails console with PostgreSQL
./docker-dev.sh test-mysql       # Run tests with MySQL
./docker-dev.sh logs             # View all service logs
./docker-dev.sh down             # Stop all services
./docker-dev.sh clean            # Remove containers and volumes

Manual Docker Compose

docker compose up db-mysql redis webapp-mysql
docker compose exec webapp-mysql bundle exec rails console

🔧 Key Features

• Separate containers for each database backend (MySQL/PostgreSQL)
• Health checks ensure databases are ready before app startup
• Volume mounting for live code reloading during development
• Environment-based configuration using ERB templates
• Comprehensive validation with automated tests
• Preserved compatibility with existing development workflow

📚 Documentation

Updated README.rdoc with comprehensive Docker setup instructions while maintaining all existing installation methods. The original development workflow remains unchanged.

✅ Validation

All Docker Compose configurations have been validated:

• Service definitions are correct and functional
• Database configurations render properly for both MySQL and PostgreSQL
• No conflicts with existing development setup
• All validation tests pass

Fixes #105.

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