Thank you for your quick response.
So if I understood it OK, docker is not able to use other alternative storage driver to access underlying system files, if you are using a COW filesystem like btrfs.
So there is no good solution, as discourse may have problems using btrfs storage driver from docker.
Reverting to ext4 is not an option, as all the migration was to be sure the data files were kept under the COW btrfs filesystem, and its ability to take snapshots, and keep data clean.
There is no point in using btrfs in other parts of the system, as its main objective is provide discourse forum access.
It is a pity because it would be great to have it running with the protection of a COW filesystem.
It is easier to use the --skip-prereqs flags. But using it in the productions system can be risky.
I have tried it in the testing machine and it works OK for now, but the problem seems to be in the long run.