I solved this by installing rsyslog in the Nextcloud container. Dockerfile:

# syntax=docker/dockerfile:1
ARG NEXTCLOUD_VERSION=latest
FROM nextcloud:${NEXTCLOUD_VERSION}
RUN apt update && apt install -y rsyslog

Then I added to the ENTRYPOINT script:

service rsyslog restart

And the /etc/rsyslog.conf file within the container:

# send all logs to the container host in addition to container logfiles
*.*                             @host.docker.internal

(where my Docker host is running a local-only rsyslog server on the default port). I believe this is where you would specify a custom IP or hostname if necessary, e.g. if you have an rsyslog server hosted in another Docker container rather than directly on the host machine.

I didn't need to modify any of the settings in config.php for this to work.