doc: how to reproduce builds

Fixes: #1755

doc/user_documentation.rst

@@ -223,6 +223,12 @@ used to create SRPM).  For more info, have a look at
 :ref:`custom_source_method`.
 
 
+Reproducing the builds locally
+------------------------------
+
+There's a separate document :ref:`reproducing_builds`.
+
+
 Temporary projects
 ------------------
 

doc/user_documentation/reproducing_builds.rst

@@ -0,0 +1,48 @@
+:orphan:
+
+.. _reproducing_builds:
+
+Reproducing Copr builds locally
+===============================
+
+In a nutshell, the build-logic in Copr is implemented as set of utilities that
+form a thin wrapper around `Mock`_.  So `Mock needs to be configured`_.
+
+Then, it should be relatively trivial to reproduce the build on your local
+machine.  The howto is dumped into every single ``builder-live.log.gz`` file
+like::
+
+    You can reproduce this build on your computer by running:
+
+      sudo dnf install copr-rpmbuild
+      /usr/bin/copr-rpmbuild --verbose --drop-resultdir --task-url https://copr.fedorainfracloud.org/backend/get-build-task/6097492-fedora-rawhide-x86_64 --chroot fedora-rawhide-x86_64
+
+Feel free to start with that workflow.  The Copr instance administrators should
+pay attention to make the reproducibility as simple as that.  But some
+situations may require additional configurations though; notable examples:
+
+    1. You might need your own Red Hat ``subscription-manager`` credentials, if
+       building against ``RHEL`` based chroots.
+
+    2. Some configuration fixes in ``mock-core-configs`` package might be
+       required (or additional configuration files added);  Copr administrators
+       might do temporary hacks around broken distribution repositories, broken
+       build methods, etc.  That's why the ``copr-rpmbuild`` utility dumps the
+       ``configs.tar.gz`` file into every single result directory.  If needed,
+       you can download the tarball and extract the appropriate piece of
+       configuration into your ``/etc/mock`` directory.
+
+    3. For non-public DistGit instances, configured typically in non-public Copr
+       instances, may need a config file in ``/etc/copr-distgit-client/``.  Ask
+       your Copr admins to provide the configuration file for you.
+
+    4. Uploaded spec files and SRPMs are very quickly removed from Copr
+       Frontend, right after the sources are imported into DistGit.  Such
+       "source builds" (not really builds in this sense) that just download
+       stuff from Copr Frontend will fail for you locally claiming that the
+       source doesn't exist, error 404.  This typically isn't a problem;  you
+       don't want to reproduce "source build", but the particular binary RPM
+       build.
+
+.. _`Mock`: https://github.com/rpm-software-management/mock
+.. _`Mock needs to be configured`: https://rpm-software-management.github.io/mock/#setup