If you want to download cracked apps and torrents avoid the iPad.
That said, I love my iPad. I have a number of pay-one-time apps that I use and I have a linux server at home for torrenting. I stream it all to the TV and occasionally to iPad. You see, iPad (iphone etc) are not made for data storage.
Start by making a simple image and make a container of it (I started with a simple python webserver)
Once you grasp basic cli commands and the docker lingo, try pulling portainer and use its web ui.
Next step would be to customize an image.
There is a lot of information on SO, and you can ask specific questions on sites like this one. YT videos are 99% shit. Avoid.
I think docker is so stupidly awesome I nowdays dont want to do anything without it.
I tried jellyfin but it was too slow to index my library. So I switched to navidrome. It works and it indexes music very fast. Searching for songs is for some reason slow. It only works with tags so browsing by folders is not possible. My workaround is a script that auto creates playlists in folders with songs from different artists/albums. Ios app play:sub is nice.
I am happy with it so far.
Firefox dockers are amazing. I have one running on my home network. I use it to access my router settings when I am away from home.