Are you still seeing this in your logs for every rebuild? If so, I suspect you were somehow looking at (thus manually renamed) the wrong directories here:
If the container’s /shared volume is mapped to /mnt/volume_tor1_01/shared/standalone on the host, then cat /mnt/volume_tor1_01/shared/standalone/postgres_data/PG_VERSION should return 13 instead of 15.
Can you share your sanitized app.yml file?