Socket.io can't be used with Reverse Proxy in serveronly mode

[Legion2]

Platform: Raspberry Pi 3 using docker as server (f9d9ff1a354d), Chrome on Windows 10 as client

Node Version: 12, the version in the latest(f9d9ff1a354d) docker image

MagicMirror Version: v2.10.1

Description: I want to use Magic Mirror behind a Reverse proxy so I can access it via a sub path in the url as http://localhost/magic-mirror/. I setup a Preserve Proxy so I can access http://localhost/magic-mirror/, but it only shows a blackscreen. The console log of the browser shows that the socket.io.js could not be loaded. That's because it's loaded from an absolute path in index.html (src="/socket.io/socket.io.js"). All other scripts are loaded without problem, because they are using relative paths.

I removed the / from the path in the index.html but socket.io also uses absolute path somewhere else. So this simple change did not solve the whole problem.

Steps to Reproduce:

1. Start the MagicMirror docker container
2. setup Reverse Proxy using e.g. apache2:

ProxyPass "/magic-mirror/" "http://localhost:8080/"
ProxyPassReverse "/magic-mirror/" "http://localhost:8080/"
ProxyPreserveHost On

3. open http://localhost/magic-mirror/

Expected Results: open http://localhost/magic-mirror/ should show magic mirror

Actual Results: Blackscreen, chrome console log:

GET http://localhost/socket.io/socket.io.js net::ERR_ABORTED 404 (Not Found)
main.js:455 Initializing MagicMirror.
...

the url should be http://localhost/magic-mirror/socket.io/socket.io.js

Configuration: default docker config

Additional Notes: I want to use Magic Mirror behind a Reverse Proxy, because then I can better manage all exposed services and protect them with password and TLS. The localhost urls are only used for simplification.

[Legion2]

I found a fix for the problem and can provide a PR. But we have to discuss the solution as there could be conflicts with the url handling and other modules.