In addition to the DB, there's also a media directory with a lot of stuff like images. I guess that should be saved as well. Not sure how that's done with Docker but I'm sure it has some mechanism to back up volumes. It's called "media" in the docker compose file, or "app/static/media/" in my non-containerized install.

While you're figuring that out: Just stop Piefed and backup all the files. This works while you're building a better solution. For my whole server with more stuff it's just a minute for the full backup and I wouldn't mind a minute of downtime in the middle of the night.

I am not much of a sysadmin either and have had to get help from some other instance admins in the past. Here is the exact process I go through to manually make and then restore a backup. I don't have this automated since I just work in dev environments, but you could put this in a shell script and schedule it with cron or something like that.

Here is how I make the backup (pgdata folder in my case):

# Prerequisite: navigate to the pyfedi folder

# Shut down the docker containers
docker compose down

# Wait for the containers to shut down...

# Then make the backup of the db volume
sudo rsync -a pgdata/ pgdata_backup

And here is how I restore the backup if I mess up my db:

# Again, need to be in the pyfedi folder

# Shut down the docker containers
docker compose down

# Wait for the containers to shut down...

# Delete the messed up db folder
sudo rm -r pgdata

# Rename the backup folder to swap it back in
sudo mv pgdata_backup pgdata

# Start the containers back up
docker compose up -d