Unable to view the apibrowser when using a subpath with nginx as a reverse proxy

[s1shed]

The apibrowser (node specific and global) cannot be viewed when nginx is configured as a reverse proxy hosting graylog under a subpath.

Expected Behavior

1. Visiting https://hostname.tld/graylog/api/api-browser/global/index.html should show the global apibrowser.
2. Clicking the node specific apibrowser button should load the apibrowser.

Current Behavior

1. The global apibrowser shows a blank page. There's a header at the top with a login form and a footer mentioning swagger, but the rest of the page is blank.
   
2. For the node specific apibrowser, with my docker-based set-up clicking the apibrowser button tries to load http://172.19.0.4:9000/api/api-browser.

Steps to Reproduce (for bugs)

1. Try to load both apibrowsers (node specific and global) when graylog is behind a reverse proxy with a subpath.

Your Environment

Graylog Version: Graylog 3.2.2 (docker tag graylog:3.2)
Elasticsearch Version: docker tag elasticsearch-oss:6.8.2
MongoDB Version: 3.6.17 (docker tag mongo:3)
Operating System: Debian unstable
Browser version: Firefox 73 & Chrome 80.0.3987.116