docs: mock's configuration

Related: https://pagure.io/koji/issue/4019

docs/source/mock_config.rst

@@ -0,0 +1,46 @@
+Interaction with Mock
+---------------------
+
+`Mock`_ is crucial component for building in Koji. Every build runs the mock to
+prepare fresh isolated environment and builds rpm or other content in chroot
+handled by Mock.
+
+For each buildroot (and thus every build) koji prepares configuration file for
+mock to be used. Content of the file depends on some administrator's decisions
+and can be tweaked by:
+
+ * ``/etc/mock/site-defaults.cfg`` on builder. This file is always consulted by
+   mock, so it is strongly recommended not to use default configuration but
+   deploy this file with more strict settings.
+   Typical values which should be changed:
+
+   - ``use_bootstrap = False`` - Bootstrap is a) not needed in most cases b)
+     slows down the build c) e.g. in combination with next option could lead to
+     unreproducible build. While this feature is pretty useful in some cases,
+     it should be turned off as default and could be selectively turned on for
+     specific buildroots per tag configuration.
+
+   - ``use_bootstrap_image`` - As noted before, this is generally unsafe
+     option, especially if used with image tags which are expected to be moved
+     (e.g. "latest"). Note, that to use this option (even via tag's
+     configuration), you need to explicitly allow it in :doc:`kojid.conf
+     <kojid_conf>` via ``mock_bootstrap_image = True``.
+
+   - ``nspawn_args`` - for old chroot, :ref:`RLIMIT_* <rlimits_kojid>` support
+     setting various RLIMIT values to the build process. In case of
+     nspawn-based chroot (``mock.new_chroot`` tag extra option) these are
+     handled by mock itself and must be set separately there.
+
+   - ``macros`` - some rpm macros could be defined every time for every build
+     based on builder itself. Typical usecase is to limit usage of CPUs on
+     given builder by setting ``config_opts['macros']['%_smp_ncpus_max'] = 2``.
+
+   - ``plugin_conf`` - Various plugins could be enabled and configured to be
+     available in build environment. Example is `rpmautospec`_ which allows
+     various ``%auto*`` macros in spec files.
+
+ * Another selective variant of altering mock's behaviour is setting its
+   options per buildtag. This is more described in :ref:`tuning-mock-per-tag`.
+
+.. _Mock: https://github.com/rpm-software-management/mock/wiki
+.. _rpmautospec: https://docs.pagure.org/fedora-infra.rpmautospec/