Simplify dev environment: inline devcontainer, drop Docker image tooling

[jslee02]

Summary

• drop the docker dev image pipeline (Dockerfiles, cache_docker workflow, build scripts/tasks) to avoid maintaining unused dart-dev images
• inline the latest Ubuntu dev image setup directly into .devcontainer/Dockerfile so VS Code/devcontainer builds locally without external images
• keep a leaner maintenance surface by removing docker build pixi tasks and scripts

Testing

• Not run (devcontainer build only)
• Run pixi run test-all to lint, build, and test your changes
• Add unit tests for new functionality (not applicable)
• Document new methods and classes (not applicable)
• Add Python bindings (dartpy) if applicable (no changes)