Interesting, it was my understanding that Docker was going in the other direction. Moving off AuFS towards Btrfs due to issues getting AuFS patches into the mainline kernel.

I'll have to look info CoreOS's reasons for going with Ext4.

I've a more or less similar script that uses overlayfs. More less than more... since it uses machined really. But it's easy to to grab commands from there and put in bocker for ex. You just need to replace the btrfs commands by the overlayfs mount, it's almost nothing :)

https://github.com/gdestuynder/mctl/ See also this terrible draft https://www.insecure.ws/linux/systemd_nspawn.html