Docker windows, How to access the host system?

Question

Apache HTTP Server

Windows server 2016
Docker for Windows version 17.12.0-ce, build c97c6d6
Образ microsoft/windowsservercore 
т.е. использование именно windows контейнеров, а не основанных на Linux

For example, if a web server ( apache or iis ) is running at localhost:81 ( 127.0.0.1:81 ) and is accessible from the internet via a static IP.
How to access it from docker container?
How to find a working internal IP host system?

Teams that don't work

Invoke-WebRequest -URI http://static_ip:81 -UseBasicParsing
Invoke-WebRequest -URI http://docker.for.win.localhost:81 -UseBasicParsing

Viewed links that did not help / did not understand

https://docs.microsoft.com/ru-ru/virtualization/wi...
https://www.ntweekly.com/2017/03/19/how-to-configu...
https://github.com/moby/moby/issues/1143