URL_BASE does not seem to work as intended

[Luux]

General information

• embetty-server version: 2.0.3 (Docker version heiseonline/embetty-server:2.0.3)
• Node.js version: 12.16.1
• Operating system name and version: Ubuntu 18.04.5 LTS (Host)
• Browser name and version: -

Steps to reproduce the behaviour

Ok, this bug did cost me hours, lol.
I installed the docker image of embetty-server to implement embetty for my homepage, which URL is called [webpage.tld] in the following examples. I configured the server so that it can be called from localhost:3000 on the host, which worked just fine. However, as soon as I wanted to use nginx to forward [webpage.tld]/embetty-server/ to [domain.tld]:3000 to handle SSL etc., I ran into 404 errors for no obvious reason.

After hours of fiddling around with different ports, nginx settings and so on, I tried to configure a subdomain "embetty-server.[domain.tld]" instead, which works just as intended. I also used nginx there for the configuration:
embetty-server.[domain.tld]:8080 -> embetty-server.[webpage.tld:3000], which works flawlessly.

My suspicion: the URL_BASE variable, which was set to [webpage.tld]/embetty-server/ did not work as expected. Therefore, the server failed at parsing for example GET /embetty-server/version, where "embetty-server" should be substituted by the given URL_BASE path. Therefore, this leads to 404 errors for [webpage.tld]/embetty-server/version (-> GET /embetty-server/version results in 404) where [webpage.tld]:3000/version (-> GET /version) just worked fine.

Expected behaviour

In the example above, GET /embetty-server/version should work just fine if URL_BASE is set to [webpage.tld]/embetty-server/

Actual behaviour

GET /embetty-server/version results in 404 errors. The GET requests arrive at the server according to the docker logs, however it seems that they are not parsed correctly.