What’s gluetun? Seems like it’s a VPN client? What’s special about it?

Gluetun can connect to multitude of VPNs, but most importantly it can be used to force other containers to use only the gluetun network, meaning if you disconnect from VPN for whatever reason, the other containers don’t suddenly send data over non-VPN network.

So if you’re torrenting and use gluetun to provide internet to the qBittorrent container, you won’t accidentally reveal your real IP if your provider’s server goes down for a few seconds.

How do you use it in your setup?

Configure it to connect to my VPN, create a file with the public port it uses, configure qBittorrent to only use gluetun for network and some script which reads the file with public port and changes it in qBittorrent.

Do I need to know about this if I use Tailscale on the host for connecting to my VPN?

Depends. I like having everything container related in the containers. Sometimes I need to do something without VPN, this would limit me. Also, if you don’t configure disconnect on VPN connection loss in a different way (interface binding), you risk revealing your IP.

Would gluetun allow me to use an additional VPN provider for certain apps without messing with the host Tailscale?

Yes. Though you would be double VPNed: App -> gluetun -> host VPN -> target server. That would probably add some latency.