as you have docker already, use the pgAdmin4 docker image, it's a tool for postgres db admin.
details here: Container Deployment — pgAdmin 4 7.6 documentation
After launching it (it creates a web admin page), add a server connection to your joplin database using the server db credentials in your original docker command for the db.
Then you can browse the database and see what its up to...
(it also helps you run the backups)
Hint: In portainer I added the pgAdmin docker container to the joplin server db network so they could talk easier...
EDIT: I should add.. messing around in there potentially is dangerous to your data ... be careful