[Docker] Replace nginx config instead of placing another file

For some reason inobvious to me, the original default.conf happened
ending up in the image from time to time.

docker/nginx/Dockerfile

 FROM nginx
 
 RUN rm /etc/nginx/conf.d/default.conf
-COPY nginx.conf /etc/nginx/conf.d
+COPY nginx.conf /etc/nginx/conf.d/default.conf
 
 RUN mkdir /var/lib/biscuit