Graylog Docker images ignore GRAYLOG_HTTP_EXTERNAL_URI for REST API Docs

## Expected Behavior
When I include `GRAYLOG_HTTP_EXTERNAL_URI=http://1.2.3.4:9100/` in environment of container I expect the API to use that URI when it serves me API documentation.

Graylog UI works as expected using that environment variable. Only REST API Docs seem to ignore it.

## Current Behavior
The container's private network IP is used instead. Can be confirmed by going to Developer Tools within browser and then
Network tab and reloading the page.

## Possible Solution
Use the environment variable value if it is provided.

## Context

Image: docker.io/graylog/graylog:5.2