mock gid

[Jitesh Shah]

I see some code in mock that picks up the mockuser uid/gid from the host
machine. why is it so? 

I ask because recently, we came across some issues while building in
koji where the builds failed with this :
DEBUG util.py:280:  Executing command: ['/usr/sbin/groupadd', '-g', '499', 'mockbuild']
DEBUG util.py:256:  groupadd: GID 499 is not unique
DEBUG util.py:319:  Child returncode was: 4
(http://arm.koji.fedoraproject.org/koji/getfile?taskID=45343&name=root.log)

GID 499 is already taken (in this case, by avahi). 
What is a good way to workaround this problem? 

Regards,
Jitesh

(PS: I am not sure whether this is the right place to ask mock-related
questions. Let me know if I need to redirect it to the mock community)