A DB container may and should be stateless, but when configured correctly the volumes specific to the storage engine are persistent. I've been running production databases in Docker since 2014 without any data loss, it makes a lot of system-level administrative work much easier.
With a healthy understanding of how the individual storage engines commit to disk, upgrading, backing up, etc. can be done in parallel and without impact to a running production system thanks to the power of overlayfs.
We do run databases that do 100k`s ops/s in containers but we don't run them in kubernetes. We just mount the VM hard drive in it.