If you VPS server isn’t KVM, or if they have a messed up kernel, it will be more difficult to support Docker & Discourse.
I know that both Digital Ocean and Vultr offer compatible VPS servers, so that’s why the recommend those.
At this point, since your VPS server has a non-standard kernel, we can’t help you. You can google for “$VPS_PROVIDER change kernel” or “$VPS_PROVIDER docker aufs”, but it’s an additional obstacle created by the server provider.