This is a followup to 71456f2
which is about generating bit-for-bit reproducible images (container/disk) that include an RPM database.

At the time, the person working on that PR was looking at RHEL8 (BDB format rpm database). We've since moved on to sqlite, which is nicer.

Except in my testing, this rpmdb.sqlite-shm is the very last thing blocking reproducible rpm-ostree container images. I hacked up coreos/rpm-ostree#4074
but this issue is to track fixing this more properly inside librpm.

I think what would be the best is if this file could be deleted entirely at the end of a build. It seems like rpm (or sqlite) will auto-recreate it in that case, but what blocks us from doing that is that things fail if the mount is read-only.