It isn’t possible to mount block storage to a service outside of DO. Block storage is meant to be used inside of DigitalOcean, and it is pretty much the same with all providers. Even if you could mount externally it would add too much latency to make it efficient.

Similarly, you can use Object Storage as an exported FS but it wouldn’t be recommended because again the latency that you would encounter wouldn’t make it efficient.

If you want to mount block storage to docker inside of a DigitalOcean droplet here is a great write up on that:
https://www.digitalocean.com/community/questions/how-to-attach-digitalocean-block-storage-to-docker-container