you can run something like this on the host - make sure you use the correct container name

#!/bin/bash
# check the container name with docker ps first
container_name="instance_name_postgres_1"

echo "dumping pgsql"
# Backup command
docker exec -t "${container_name}" pg_dumpall -c -U lemmy > lemmy_dump.sql

suggest you not leave unencrypted backup on the host system but copy it somewhere else, preferably after encrypting it with gpg.