use global option to enable bootstrap-image

builder/kojid

@@ -302,7 +302,10 @@ class BuildRoot(object):
                 opts['tag_envvars'][key[8:]] = self.config['extra'][key]
         if 'mock.bootstrap_chroot' in self.config['extra']:
             opts['bootstrap_chroot'] = bool(self.config['extra']['mock.bootstrap_chroot'])
-        opts['bootstrap_image'] = self.config['extra'].get('mock.bootstrap_image')
+
+        # it must be allowed in kojid.conf *and* in tag's extra info
+        opts['bootstrap_image'] = self.options.mock_boostrap_image and \
+            self.config['extra'].get('mock.bootstrap_image')
 
         output = koji.genMockConfig(self.name, self.br_arch, managed=True, **opts)
 
@@ -6400,6 +6403,7 @@ def get_options():
                 'use_createrepo_c': True,
                 'createrepo_skip_stat': True,
                 'createrepo_update': True,
+                'mock_boostrap_image': False,
                 'pkgurl': None,
                 'allowed_scms': '',
                 'scm_credentials_dir': None,

builder/kojid.conf

@@ -116,6 +116,9 @@ from_addr=Koji Build System <buildsys@example.com>
 ;if set to True, failing subtask will not automatically cancel other siblings
 ;build_arch_can_fail = False
 
+;if set to True, tag extra 'mock.bootstrap_image' can be used
+;mock_boostrap_image = False
+
 ;image build with raw-xz type will use following xz options
 ;xz_options=-z6T0
 

docs/source/using_the_koji_build_system.rst

@@ -407,6 +407,9 @@ environment follows:
   before using this. You could need it, but do it with following
   recommendations:
 
+  - you need to explicitly allow builders to do that (``mock_boostrap_image =
+    True`` in ``kojid.conf``).
+
   - you need to have builders with `podman <https://podman.io/>`_ installed and
     working.