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.

docs/Plugin-Export-Buildroot-Image.md

@@ -0,0 +1,64 @@
+---
+layout: default
+title: Plugin export_buildroot_image
+---
+
+This plugin allows you to (on demand) export the Mock chroot as an OCI image in
+local archive format (tarball).  This tarball can provide additional convenience
+for local build reproducibility.  See the example below for details.
+
+By default, this plugin is **disabled**.  You can enable it using the
+`--enable-plugin export_buildroot_image` option in `--rebuild` mode.
+
+This plugin has been added in Mock v6.0.
+
+## Example use-case
+
+First, let's start a standard Mock build, but enable the OCI archive generator:
+
+    $ mock -r fedora-rawhide-x86_64 --enable-plugin export_buildroot_image \
+            /tmp/quick-package/dummy-pkg-20241212_1114-1.src.rpm
+    ... mock installs all build-deps, and does other chroot tweaks ...
+    Start: producing buildroot as OCI image
+    ... mock performs the rpmbuild ...
+    INFO: Results and/or logs in: /var/lib/mock/fedora-rawhide-x86_64/result
+    Finish: run
+
+The archive has been saved in the result directory:
+
+    $ ls /var/lib/mock/fedora-rawhide-x86_64/result/*.tar
+    /var/lib/mock/fedora-rawhide-x86_64/result/buildroot-oci.tar
+
+You may use this tarball together with [the `--buildroot-image` option
+then](Feature-buildroot-image).  But also, you can try re-running the
+build without Mock, like this:
+
+    $ chmod a+r /tmp/quick-package/dummy-pkg-20241212_1114-1.src.rpm
+    $ podman run --rm -ti \
+        -v /tmp/quick-package/dummy-pkg-20241212_1114-1.src.rpm:/dummy-pkg.src.rpm:z \
+        oci-archive:/var/lib/mock/fedora-rawhide-x86_64/result/buildroot-oci.tar \
+        rpmbuild --rebuild /dummy-pkg.src.rpm
+
+    Installing /dummy-pkg.src.rpm
+    setting SOURCE_DATE_EPOCH=1401926400
+    Executing(%mkbuilddir): /bin/sh -e /var/tmp/rpm-tmp.XIm441
+    ...
+    Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.pqJ9hu
+    ...
+    Executing(%build): /bin/sh -e /var/tmp/rpm-tmp.iaeMZG
+    ...
+    Executing(%install): /bin/sh -e /var/tmp/rpm-tmp.SHktaE
+    ...
+    Processing files: dummy-pkg-20241212_1114-1.fc42.x86_64
+    ...
+    Executing(%clean): /bin/sh -e /var/tmp/rpm-tmp.E71FWH
+    ...
+    + exit 0
+
+**Warning:** This method of reproducing a Mock build is not recommended for
+production use.  During a normal/full Mock rebuild, Mock ensures the buildroot
+is fully up-to-date.  Using just plain `rpmbuild` within Podman may result in
+outdated files, different structure in the kernel-driven filesystems like
+`/proc`, `/dev`, and `/sys`, different SELinux assumptions, permissions, etc.
+Proceed with caution, and be prepared to encounter some differences (and perhaps
+different build failures).