I thought this video was rather interesting, because at 12:27, the presenter crunches the numbers to find out how many years it would take for a new computer purchase to be more environmentally friendly (in regards to total CO2 expended) compared to using a less efficient used model.
Depending on the specific use case, it could take as little as 3 years to breakeven in terms of CO2 if both systems were at max power draw forever, and as long as 30 if the systems are mostly at idle.
Kodi install instructions are here
I don’t use docker, I use lubuntu with normal packages. So for example Kodi is just installed from the Team Kodi PPA repository (which, granted, is outdated, but it works fine and I don’t need the latest and greatest) and just set it up to be auto-started when X starts so that on the TV it’s as if Kodi is the interface of that machine.
Qbittorrent is just the server only package (qbittorrent-nox) which I control remotelly via its web interface and the rest is normal stuff like Samba.
After the inital set up, the actual linux management can be done remotelly via ssh.
That said, LibreELEC is a Linux distro which comes with Kodi built-in (it’s basically Kodi and just enough Linux to run it), so assuming it’s possible to install more stuff in it might be better - I only found out about it when I had my setup running so never got around to try it. LibreELEC can even work in weaker hardware such as a Raspberry Pi or some of its clones.
Also you can get Kodi as a Flatpak which works out of the box in various Linux distros so if you need the latest and greatest Kodi plus a full-blown Linux distro for other stuff you might do the choice of distro based on supporting flatpack and being reasonably lightweight (I actually originally went for Lubuntu exactly because it uses a lightweight Window Manager and I expected that N100 mini-pc to need it, though in practice the hardward can probably run a lot more heavy stuff than that, though lighter stuff means the CPU load seldom goes up significativelly hence the fan seldom turns on and so the thing is quiet most of the time and you only hear the fan spinning up and then down again once in a while even in the Summer).
As for docker, there are a lot of instructions out there on how to install Kodi with Dockers, but I never tried it.
Also you might want to get a remote like this, which is a wireless remote with a USB adapter, not because of the air-mouse thing (frankly, I never use it) but simply because the buttons are mapped to exactly the shortcuts that Kodi uses, so using it with Kodi in Linux is just like using a dedicated remote for a TV Media Box - in fact all those thinks are keyboard shortcuts (that remote just sends keypresses to the PC when you press a button) and they keyboard shortcuts for media players seem to be a standard.
One advantage of a raspberry pi is the possibility of using the TV remote to control Kodi due to HDMI CEC compatibility.
What are you using for storage?