Set the $releasever variable in Mock config.

conf/mock.cfg

@@ -4,7 +4,7 @@ config_opts['legal_host_arches'] = ('$arch',)
 config_opts['chroot_setup_cmd'] = 'install $group'
 config_opts['dist'] = ''
 config_opts['extra_chroot_dirs'] = [ '/run/lock', ]
-config_opts['releasever'] = ''
+config_opts['releasever'] = '$releasever'
 config_opts['package_manager'] = 'dnf'
 config_opts['nosync'] = True
 config_opts['use_bootstrap_container'] = False

module_build_service/builder/MockModuleBuilder.py

@@ -91,6 +91,7 @@ class MockModuleBuilder(GenericBuilder):
         self.config = config
         self.groups = []
         self.enabled_modules = []
+        self.releasever = module_build_service.utils.get_local_releasever()
         self.yum_conf = MockModuleBuilder.yum_config_template
         self.koji_session = None
 
@@ -257,6 +258,7 @@ class MockModuleBuilder(GenericBuilder):
                 self.groups = config_opts["chroot_setup_cmd"].split(" ")[1:]
                 self.yum_conf = config_opts['yum.conf']
                 self.enabled_modules = config_opts['module_enable']
+                self.releasever = config_opts['releasever']
 
     def _write_mock_config(self):
         """
@@ -271,6 +273,7 @@ class MockModuleBuilder(GenericBuilder):
             config = config.replace("$group", " ".join(self.groups))
             config = config.replace("$yum_conf", self.yum_conf)
             config = config.replace("$enabled_modules", str(self.enabled_modules))
+            config = config.replace("$releasever", str(self.releasever))
 
             # We write the most recent config to "mock.cfg", so thread-related
             # configs can be later (re-)generated from it using _load_mock_config.

module_build_service/utils/general.py

@@ -452,6 +452,16 @@ def import_fake_base_module(nsvc):
         import_mmd(session, mmd)
 
 
+def get_local_releasever():
+    """
+    Returns the $releasever variable used in the system when expanding .repo files.
+    """
+    # Import DNF here to not force it as a hard MBS dependency.
+    import dnf
+    dnf_base = dnf.Base()
+    return dnf_base.conf.releasever
+
+
 def import_builds_from_local_dnf_repos():
     """
     Imports the module builds from all available local repositories to MBS DB.