With DefaultHostname=%HostName% in webone.conf, the docker container's internal IP addresss is used, rather than the container's hostname.

Started the docker container using argument --hostname mytest
A shell in the container shows that it is respected in the environment

# hostname
mytest

Also tested with a small program built inside the container, which printed the hostname mytest

using System;

class Program
{
    static void Main()
    {
        Console.WriteLine($"Machine Name: {Environment.MachineName}");
    }
}

Webone is running, but the proxy is using the container's internal IP rather than the hostname provided. You can see this when looking in the network tab of your browser:

Also the default page shows similar problems:

The problem might be in lines 218-226 in ConfigFileLoader, probably in line 220, though I can't see how.