Add support for btrfs storage driver in docker

Add support for btrfs storage driver in docker

After several months of using btrfs in our forum in production with no issue, I can say that btrfs seems quite stable and I am proposing to add it to the supported storage drivers under docker.

Using btrfs makes it a breeze to take snapshots and revert to a previous state of discourse, and taking backups with minimal forum interrupt time.

Here is a discussion about it in discourse forum:
https://meta.discourse.org/t/unable-to-rebuild-app-not-supported-docker-storage-driver-btrfs/209200/14