I have all of mine on the same (or accessible) internal LAN so they can all talk to each other. You can get the connection going with Wireguard if they are in different places in terms of networking.
Amounts to the same thing but no. Promox servers with two bridged interfaces. One interface has a public IP, the other a 10.0.10.0/24 etc. Multiple baremetal servers are connected by wireguard and have access to each other's private subnets. Like one other might be the 10.0.20.0/24. Setup the routes and good to go. Firewall to taste. My private LAN is all open.
This is not just for docker. There are other vms and lxc containers too.