What's confusing here? Break down the steps and parts of the command

It's not their job to teach you how to use docker.

Yeah, I can't fuck with Docker either. Check out Yunohost if you want something that is actually easy to get up and running.

I have a 2 page Google Doc that I wrote while installing Matrix (because I wanted to be able to recover from a complete system loss, and knew I'd forget what I did). Half of the doc is my HAProxy notes.

Are you still having issues? I could try cleaning up my notes for a wider audience (note: my professional background includes technical writing and corporate technical training, so I'd be super anal about, and it would take a few hours at least).

Is this meant to be single-user, or a larger host?

Docker

There's your first mistake.

Oh, hi, I'm just stopping by from the 'compile from source and create a systemd unit file' tribe.

So you added the secret to the file and restarted the docker container, right?

Something that I think will help you with self-hosting in the future is to always read through the entire process for setting up whatever you want to set up first, beginning to end, so that you are familiar with what you need to do before attempting it the first time. It's helped me numerous times myself.