Hi guys,
Thought I'd register and chime in.
I am myself a new unRAID user, purchased a license on day one, it's got too much going for it, I'm enjoying the unRAID experience so far...
Anyway, to the problem at hand...
I too started on the journey of setting up docker and an ARK server in docker, however after some trial, error and research through these forums, it cannot be done unfortunately with docker on unRAID (6).
The game server files needs to be downloaded, updated and maintained by Valve's SteamCMD, which is unfortunately 32-bit only, but unRAID doesn't include support for running any 32-bit software on it directly or through docker.
To set one up, you'll need to set up a new VM on your unRAID server, set up your favourite Linux OS in the VM, install docker and run it manually from there I'm afraid.
I hope that helps.