[FeatureRequest]: Support to run behind Proxy with relative paths.

[sergioamr]

Is your feature request related to a problem? Please describe.

In our current setup we have around 60 Duets which are coordinated through a common interface, with relative paths in the following format:

<SERVER_IP_ADDRESS>/FARM1/ARM1/
<SERVER_IP_ADDRESS>/FARM2/ARM2/
...

In order to run it through a proxy server, we encounter several issues:
First the file is already compressed and we had to use the following for the proxy to work. (not really an issue anymore, since we managed to make it work by deflating the GZ distributed file, and it might help someone to proxy)

<IfModule mod_proxy_html.c>
    ProxyPass /arm1/ http://192.168.104.1:10001/
    ProxyPassReverse / http://192.168.104.1:10001/
    <Location /arm1/>
        ProxyHTMLEnable On
        SetOutputFilter INFLATE;proxy-html;DEFLATE

        ProxyHTMLCharsetOut *

        ProxyHTMLURLMap / http://192.168.104.1:10001/
    </Location>
</IfModule>

Our problem comes from the web sockets and commands to rr_reply and rr_model, which have their paths hardcoded by process.env.BASE_URL.

Describe the solution you propose.

Would there be an option to make it work with relative paths, or append a GET parameter ?root=/NEW_PATH/

Describe alternatives you've considered

I cannot find an alternative, only writing a patch and diverge from the main branch.

Provide any additional context or information.