If you want to use your own (custom) nginx.config, then just place it in your local data volume you started the docker container with like /home/pi/tasmoadmin/data/nginx/ and replace existing nginx.config.

(Re-)Start your container to use custom nginx.config.

The default config is found here: nginx.conf

To download and save the default config at the current dir:

wget https://raw.githubusercontent.com/reloxx13/TasmoAdmin/master/.docker/rootfs/etc/nginx/nginx.conf