sed inline replacement not working from Dockerfile
The problem is in the:
VOLUME /etc/cups/After that line, any changes to /etc/cups may not be seen in the final container (some single file copy commands do apply, so it's not perfect).
Either move your volume line to the end, or preferably remove it from your image entirely. These entries block the ability to extend your image with changes to this folder later. And you can always make your own volume with the docker run -v ... where you want it (or in your docker-compose.yml). Creating volumes in the image means you'll get anonymous volumes listed in docker volume ls after running your image.