I've got docker installed and I've got a single container running (vaultwarden) via the docker pull/docker run commands provided on its Github page. I'm not sure how to learn more about that container or how to build (compose?) others myself.
I'd like to learn how to transition nginx and pihole to containers and network them together. I'd also like to add paperless-ngx to my setup.
(note: I'm very new to Linux in general, mostly rpis, poking around with debian now)
Turning a program into a container probably isn't what you're looking for. Unless you're looking for an obscure program, someone has probably done it already.
Get accustomed to using docker compose instead of docker. It's basically just saving the commands as a text file (YAML file) to keep it all persistent.