@Captain I was struggling with the same issue as you but just got it to work.
On my x86 server I also run OpenMediaVault (OMV5, so Debian Buster) and using Portainer to manage an array of docker images. Beets had been running nicely in the cointainer from Linuxserver.io for some time, until a container update broke the image. I only noticed it after a while later because the update was done automatically via Watchtower. (I know it’s not reccomended by the kind people at Linuxserver.io, but it helps me stay up to date on other security patches without much active server management.)
I had the same python 3.8 errors and had been struggling to get beets working again. After reading this thread I added the :nightly tag to the docker-compose stack in portainer like so:
image: ghcr.io/linuxserver/beets:nightly
After that beets worked again as expected, on OMV5 with Portainer, beets version reports:
beets version 1.5.0
Python version 3.8.10
Hopefully the python patch gets merged into a new release of the container soon.