How do I set user, group, and permissions, with docker-compose

I'm using docker-compose for the first time and trying to setup this project: https://github.com/seanap/auto-m4b

I've got it working mostly. The container finds my .mp3 files and converts them to an .m4b, but the new file it outputs has the wrong user, group, and permissions.

I've tried editing the .yml file and putting PUID, PGID, and UMASK, under the environment section with no success.

Here is my compose file:

version: '3.7'
services:
  auto-m4b:
    image: seanap/auto-m4b
    container_name: auto-m4b
    volumes:
      - /mnt/user/appdata/auto-m4b:/config
      - /mnt/user/data/torrents/audiobooks_temp:/temp
    environment:
      - PUID=99
      - PGID=100
      - UMASK=002
      - CPU_CORES=2
      - SLEEPTIME=1m

I don't have anything configured on the .env file.

Any help would be greatly appreciated. I feel like it's probably something simple I'm missing.

If this is the wrong place to ask for help with docker-compose, I apologize, please point me in the right direction. Thank you!