You should open a thread to investigate and fix that.
Or maybe it’s time to consider using Home Assistant OS.
That looks to match what you have in your compose file. I wonder if you’ve already passed it through to some other container - or if you’re running Docker rootless.
Could the problem be that I’m running Docker Desktop?
Yes, Docker desktop does not allow device pass-through.
Can I pass through a USB device to a container?
Unfortunately, it is not possible to pass through a USB device (or a serial port) to a container as it requires support at the hypervisor level.
So, to achieve what I’m trying to do, should I uninstall Docker Desktop and install Docker CE?
Yes, indeed
Do you have any advice on the cleanest way to go about doing that? Is there a best practice when it comes to uninstalling Docker Desktop so that nothing remains to mess with the CE install? Will I be able to keep my compose files and just re-map my directories to new containers?
How did you install docker desktop ?
I believe I just did it through Terminal. I’m running Ubuntu on an old office pc I saved from the trash.
Ah… FFS
Yeah, there’s a reason that the docs talk about actual Docker 
Docker Desktop is very different.
Just install Docker: Install Docker Engine on Ubuntu | Docker Docs
I’m sorry, I probably should have been more specific at the outset. 
Okay, so, long overdue update but I haven’t had any time to actually work on this. I completely uninstalled Docker Desktop and replaced it with Docker Engine. I even installed Portainer. I have Home Assistant reinstalled, but for some reason, the 8123 port isn’t exposed, and I don’t know how do set it to work, aside from adding
ports:
- "8123:8123"
to the compose file. Am I missing something? I feel like I’m on the right path and that this should be the easiest fix so far.
UPDATE: Well, that fixed itself, as far as I can tell. I just had to give it a little more time to spin up.
UPDATE 2: It’s all working! I wish I had known to start with Docker Engine, but thank you for all the help, everyone!