Can't change FUNKWHALE_API_PORT in docker setup, breaks API

Steps to reproduce

1. Follow the docker install guide at https://docs.funkwhale.audio/administrator/installation/docker.html up to editing .env
2. In .env, set FUNKWHALE_API_PORT to anything but 5000, for example, 3456
3. Finish following the guide and set up a reverse proxy to forward to 127.0.0.1:3456

What happens?

Frontend is accessible via the reverse proxy, but all API requests return 502 errors. It's not possible to log in.

In the docker logs, the frontend complains that it cannot connect to the api at [some internal docker address]:3456, says connection refused.

What is expected?

Frontend is accessible via the reverse proxy and logging in works fine.

Context

Funkwhale version(s) affected: 1.3.1

I needed to change which external port funkwhale listened on, but a comment in docker-compose.yml prompted me to change that variable in .env rather than change the port mapping directly. It seems that changing the variable changes the port mapping and which port the frontend looks for the API at, but not which port the API listens on.

I worked around this by just editing the port mapping in docker-compose.yml like I was going to in the first place.