Add a base url for proxies

Hey,

I'm running this using the docker image and nginx. I use nginx to reverse proxy the requests to the docker image. Unfortunately you guys use absolute paths and have no means on how to set the base url for the app. So the assets cannot be loaded as they mismatch to / while the app is actually being served from /beta/ with a location match.

For reference gogs lets you set the base url in the config: https://github.com/gogits/gogs/issues/1844

Proxy config:
```
location /beta/ {
proxy_pass http://127.0.0.1:3000/;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto https;
proxy_redirect off;
}
```

Ideally one could set this using a ENV variable. Alternatively you will use relative paths - might not be the smartest solution.

No source file or test is named. Start by inspecting how the Docker image configures the app's URL and how asset paths are generated, then reproduce the nginx setup under /beta/. Done means the application and its assets load correctly from a configurable subpath, ideally using an environment variable.

Written by the indexing model from the issue text.