We will continue to support Oracle Container Runtime for Docker on Oracle Linux 8 after the UEK6 is released for OL8 which will restore btrfs support when you boot into the UEK. Until that occurs, we recommend staying on Oracle Linux 7 and using UEK5 with Docker.
If I may say so, and this may sound odd and like a travesty to anyone betting on Solaris, but Oracle Linux seems to care more about "sustainable technology" than Redhat. However, Apple and Microsoft, who I'd say used to care, a long time ago, are perhaps the worst-case scenario, at least for IT professionals. Now since Redhat is IBM, I guess we'll have to see what's next.