CMD_URL_PATH not working

I've set `CMD_URL_PATH` to `pad/` (also tried `pad`) in my `docker-compose.yml`, since I want to proxy from a different subdir.

This works partly; the screenshot for instance is loaded from https://my.domain.tld/pad//screenshot.png, but all other media is loaded from the root, e.g., https://my.domain.tld/build/cover-styles-pack.css.

I'm using NGINX, see the following config excerpt:

```nginx
location ^~ /pad/ {
proxy_pass http://127.0.0.1:12345/;
include proxy_params;

proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_cache_bypass $http_upgrade;
proxy_redirect off;

add_header X-Pad "test";
}
```

It's a pretty standard websocket-enabled proxy config.

I'm not sure if it's an issue of the infra in this repo (often, env vars are translated into config files by startup scripts), or needs to go into https://github.com/codimd/server.