So, quick testing is that mpv will pull subtitles by default but not display them without cycling the track (with J) or setting slang with an available language code[s]. (with slang=en,en-US for English.) Although, it will completely ignore auto-generated subtitles.
I did find that ytdl-raw-options='sub-langs="en,en-US",write-sub=,write-auto-sub=' will also give the option to use auto-generated subs.
I'm not too familiar with SMPlayer but I was seeing that you can use an mpv.conf with it.
The issue is that the docker service runs as root and their defaults added the user to a group that allows them to control that service without
sudoSo, the root filesystem can just be passed as a volume to a container and then do whatever you want from there.