Add Docker and AI agent configuration for multi-Rails testing

[takaokouji]

This commit sets up the infrastructure for testing jsonapi-resources across multiple Rails versions (6.1, 7.0, 7.1, 7.2, 8.0, 8.1).

Changes:

• Add Dockerfile with Ruby 3.2 for multi-version testing
• Add docker-compose.yml with services for each Rails version
• Add .dockerignore to optimize Docker builds
• Add CLAUDE.md with comprehensive architecture and development docs
• Add .serena configuration for AI agent tooling
• Update .gitignore to exclude log/*.log files

Docker usage:
docker-compose run rails-6.1 # Test with Rails 6.1.7.10 docker-compose run rails-7.0 # Test with Rails 7.0.10
docker-compose run shell # Interactive shell

🤖 Generated with Claude Code

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions