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.
> 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.
No, you couldn't, and no, you wouldn't.
To build a swarm you need a lot of fiddling and tooling. Where are you keeping them? How are they all connected? What's the clustering software? How is this any better than an old PC with a few SSDs?
Raspberry Pi with any amount of RAM is an exercise in frustration: it's abysmally slow for any kind of work or experimentation.
Really, the only useful advice is to use an old PC or use a VPS or a dedicated server somewhere.
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.