chore: remove root dockerignore file

No longer needed in monorepo structure:
- No CI/CD builds Docker images from root
- Dockerfiles are in packages/core (dev only) and templates
- Templates include their own dockerignore files
- Users build from their own projects, not this repo

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: boyney123