pory

Kopia(UI) is a client you run on your system, yes. It’s not containerized or web-managed, though I think there might be a web UI you could host for managing the CLI version. The Docker container on your pal’s setup would just be presenting you with a network folder that you point the backup system at.

If you want something a bit more reliable than Duplicati with a similar list of supported platforms, look into Kopia, its GUI (KopiaUI) is a lot more convenient if you’re not used to the command line. I find restore ops to be faster than Duplicati, and unlike Duplicati I’ve never experienced database corruption. For opening a webDAV instance that you can use as a data bucket for your files remotely, I’ve been very impressed with Copyparty.