We have this code in koji_build_msg.py:

        # FIXME This is probably a temporary approach, could change in future, see D1002#19239
        if owner == 'containerbuild':
            return self._process_docker(msg)
        else:
            if 'fc' not in release:
                raise exc.TriggerMsgError('rejecting message for not containing "fc" in release '
                                          '(%s)' % nvr)
            return self._process_package(msg)

It seems the owner=='containerbuild' approach is no longer (fully) valid. This task failed:
https://taskotron.fedoraproject.org/taskmaster/builders/all/builds/295213
because there are no RPMs in this Koji build:
https://koji.fedoraproject.org/koji/buildinfo?buildID=1141463
because it's a container build. But we scheduled it anyway. Because it's not owned by containerbuild.

Here's the relevant fedmsg:
https://apps.fedoraproject.org/datagrepper/id?id=2018-40fb4c85-8214-4d74-a403-aadb304b2d69&is_raw=true&size=extra-large

We need to find a different way of filtering non-rpms out of koji fedmsgs, or we need to request such an option to be implemented (a ticket against koji, most probably).