Hallo,
i solved the problem by changing the docker-entrypoint.sh script directly in the nginx docker container.
Follow these steps:
Step 1 - enter the container:
docker exec -it zammad-docker-compose_zammad-nginx_1 /bin/bash
Step 2 - install vi editor:
apt-get update && apt-get install vim
Step 3 - edit the config file:
vi /docker-entrypoint.sh
change in the # configure nginx section the option directly to https and save the file (like this):
sed -e "s#proxy_set_header X-Forwarded-Proto .*;#proxy_set_header X-Forwarded-Proto https;#g" \
Step 4 - Leave and restart container
docker container restart zammad-docker-compose_zammad-nginx_1
These steps worked for me