If you already know Debian that’s a big point in its favor. Nothing beats a distro you’re familiar with. (And I’d make the same argument if you were used to Ubuntu.)

I’ve used both Debian and Ubuntu Server on my home servers and I ended up returning to Debian.

• Debian embodies and follows the most essential Linux values. It’s open, it’s run by an open organization, it avoids proprietary stuff.
• Debian is long-lived. It’s been around for 30 years, it’s mature, self-sustaining, community-run project, and it will be around 30 years from now. Whereas I wouldn’t bet on Canonical being around in 30 years.
• The upgrade path for Debian is better than for Ubuntu Server LTS.(1) You can upgrade it in-place between major versions a lot more reliably and painlessly than Ubuntu.
• Debian is dedicated to being stable. (Please run stable, btw.)

I’m prepared to state that Ubuntu is Debian, just with more quirks. It offers nothing essential that you can’t do on Debian, and it will just complicate your life when the LTS support period eventually runs out.

(1) You can fuck up both Ubuntu and Debian’s upgrades by adding a lot of 3rd-party repos because apt doesn’t safeguard against external repos interfering with the core repos’ dependency graph.

So the trick is to keep the OS minimal, install only Docker from its repo and install anything else in Docker containers. That way you benefit the most from Debian being stable and very little from Ubuntu Server.

If you also need to run system containers and virtual machines you can add Incus later to the mix alongside Docker and still keep your host OS lean and simple.

You can also consider completely migrating to Proxmox later, which is also Debian under the hood but it’s a more turnkey solution. I wouldn’t recommend jumping straight into Proxmox unless you’re fairly sure that you’d need to run VMs. (If you’re unsure stick to plain Debian for now.)