docker-in-docker setup does unsynchronized cert creation?

[danwinship]

extended_networking_minimal has started occasionally failing provisioning because the master fails to start, logging:

Feb 27 09:19:28 nettest-master openshift-generate-master-config.sh[151]: F0227 09:19:28.617473     182 start_master.go:111] serial file /data/openshift.local.config/master/ca.serial.txt out of sync ram=9 disk=10

This is a new error added in #18405 with the comment

// do a best effort check to make sure concurrent external writes are not occurring to the underlying serial file

so, presumably, dind-setup is screwing this up