Add Docker support for multi-version Rails testing

[takaokouji]

Summary

This PR adds Docker support for testing across multiple Rails versions, making it easy to test the gem with different Rails versions in isolated environments.

Features

• 🐳 Docker Compose configuration with services for Rails 6.1, 7.0, 7.1, 7.2, 8.0, and 8.1.2
• 📦 Automated dependency management per Rails version
• 🔄 Shared bundle cache for faster builds
• 🛠️ Interactive shell mode for debugging
• 🧹 Optimized .dockerignore for faster builds

Usage

# Test specific Rails version
docker-compose run rails-7.0
docker-compose run rails-8.1

# Interactive shell with specific version
RAILS_VERSION=8.0.4 docker-compose run shell

# Run tests inside container
docker-compose run shell
bundle exec rake test

Docker Services

• rails-6.1 - Rails 6.1.7.10
• rails-7.0 - Rails 7.0.10
• rails-7.1 - Rails 7.1.6
• rails-7.2 - Rails 7.2.3
• rails-8.0 - Rails 8.0.4
• rails-8.1 - Rails 8.1.2
• shell - Interactive shell (default: Rails 6.1)

Files Added

• Dockerfile - Container image with Ruby 3.2 and dependencies
• docker-compose.yml - Service definitions for each Rails version
• .dockerignore - Optimized build context

Dependencies

This PR depends on #1480 (Rails 7.1+ support) and should be merged after it.

🤖 Generated with Claude Code

[takaokouji]

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