fix: docker build backend context

[tspicer]

Summary

• Include build_package.py in the Docker build context and copy it before the project install layer.
• Add CI workflow regression checks for the Dockerfile and .dockerignore expectations.
• Refresh the lockfile after the FastMCP/code-mode dependency update so uv sync --frozen remains valid.

Root Cause

pyproject.toml uses build_package.py as an in-tree PEP 517 backend, but the Docker build context excluded that file and the Dockerfile did not copy it before uv sync built the project. The project install layer therefore failed with ModuleNotFoundError: No module named 'build_package'.

Validation

• uv lock --check
• uv run ruff check --fix tests/unit/test_ci_workflows.py
• uv run pytest tests/unit/test_ci_workflows.py tests/unit/test_stage_wheel_resources.py
• docker build -t amazon-ads-mcp:latest .