Hi,

if you have a grudge against our infra team, OBS is the best option.
More seriously, OBS has a major flaw: it's a pain to deploy or update and we need to have people able to fix bugs in a rails app.

I advise you to discuss this with infra team before considering further this option.

One quick remark:
> build package in VM, which is safer then Koji (just chroot in Koji)

Safer, but it has an overhead. I'd rather add LXC support to Koji (much less overhead and pretty much as safe as heavier virtualization solutions).

best regards,
H.