enhance: Add comprehensive .gitignore for deb-mock project

- Add mock-specific build artifacts (chroot/, mock-*, mockroot/)
- Include package build files (*.deb, *.changes, *.buildinfo)
- Add development tools (.coverage, .pytest_cache, .tox)
- Include system files (.DS_Store, Thumbs.db, ._*)
- Add temporary and backup files (*.tmp, *.bak, *.backup)
- Include local configuration overrides (config.local.yaml, .env.local)
- Add test artifacts and documentation builds
- Comprehensive coverage for Python build system project

This ensures build artifacts, chroot environments, and development
tools are properly ignored in version control.

behave/README.md

@@ -0,0 +1,14 @@
+BDD for Mock
+============
+
+This test-suite can destroy your system!  Not intentionally, but some steps
+require us to use root (e.g. install or remove packages).  **Never** execute
+this test suite on your host system, allocate some disposable machine.
+
+How to run the tests
+--------------------
+
+1. Install the Mock RPM that you want to test.
+
+2. Run `$ behave` command in this directory, with `--tags tagname` if you want
+   to test only subset of all provided scenarios.