What ever you have laying around is a great starting point.
It all comes down to what you want to spend vs what you want to host and how you want to host it.
You could build a raspberry pi docker swarm cluster and get very far. Heck, a single Pi 5 with 4gb of memory will get you on your way. Or you could use an old computer and get just as far. Or you could use a full blown rack mount server with a real IPMI. Or you could use a VPS and accomplish the same thing in the cloud.
It all comes down to what you want to spend vs what you want to host and how you want to host it.
You could build a raspberry pi docker swarm cluster and get very far. Heck, a single Pi 5 with 4gb of memory will get you on your way. Or you could use an old computer and get just as far. Or you could use a full blown rack mount server with a real IPMI. Or you could use a VPS and accomplish the same thing in the cloud.