fix: Add Python artifacts to .dockerignore

Docker build fails when local .venv directory exists because:
1. COPY . /workspaces/serena/ copies .venv into container
2. RUN uv venv fails with 'virtual environment already exists'

This change prevents the issue by excluding Python development artifacts:
- .venv (virtual environment)
- __pycache__ (Python bytecode cache)
- *.pyc (compiled Python files)
- .pytest_cache (test cache)

These files are not needed for Docker builds as the container creates
its own clean environment.

Co-authored-by: Cursor <cursoragent@cursor.com>

.dockerignore

@@ -7,3 +7,9 @@ docs/.jupyter_cache
 docs/_build
 coverage.xml
 docker_build_and_run.sh
+
+# Python artifacts (prevents Docker build conflicts when .venv exists locally)
+.venv
+__pycache__
+*.pyc
+.pytest_cache