Improve README about nginx-proxy-subdirectory

[theit8514]

TBH, I don't really use this all that much anymore. I previously had a single IP + single host SSL certificate and wanted to secure multiple docker containers behind it. The setup I use now no longer requires this project.

I found my old init files for the docker containers I used then. Here is the setup process for the nginx container:
docker run -d --name nginx-proxy-subdirectory --restart=always -p 19080:80 -p 19443:443 -e EXTERNAL_HTTPS_PORT=19443 -v /var/run/docker.sock:/tmp/docker.sock:ro -v /volume1/docker/nginx-proxy-subdirectory/certs:/etc/nginx/certs -v /volume1/docker/nginx-proxy-subdirectory/vhost.d:/etc/nginx/vhost.d theit8514/nginx-proxy-subdirectory

This container runs both a HTTP and HTTPS port, and the HTTP port redirects to 443 by default. The EXTERNAL_HTTPS_PORT setting changes the port that is used for the redirect (if you use NAT and don't use external port 443, then this should be whichever port you have your NAT set to).

Then for each of the containers hosting services, I would add environment variables to which subdirectory that container should be hosted at:
-e VIRTUAL_PORT=8081 -e NGINX_PROXY_SUBDIRECTORY=my_directory -e NGINX_PROXY_SUBDIRECTORY_MAP_TO_ROOT=0

If I accessed the host via https://localhost:19443/my_directory/, nginx would proxy the request to the docker container that has NGINX_PROXY_SUBDIRECTORY=my_directory. The VIRTUAL_PORT setting determines which port to use on the container, and NGINX_PROXY_SUBDIRECTORY_MAP_TO_ROOT setting determines if the subdirectory is appended to the internal request. In this case, the request would be proxied to http://<docker container ip>:8081/my_directory/