How to tone down Gitlab logging for small installation

So I’m trying to set up a Gitlab instance for personal use on my Synology NAS. I got it up and running using docker pretty easily, but there is a snag: in the default configuration continually generates log output every couple of seconds. This consumes about 2 MB per hour. The diskspace is actually not my main concern, the constant log output prevents the NAS from going into sleep mode.
According to the documentation I can set the log level using an environment variable. I tried setting it to “warn” but that has very little effect, if any.
Any tips how I can get Gitlab to restrict logging to the time periods when I’m actually using it would be appreciated.
Here is the docker command I use to start gitlab:

export GITLAB_HOME=/volume1/docker/gitlab

sudo docker run --detach \
  --hostname gitlab.shire.lan \
  --publish 30443:443 --publish 30080:80 --publish 30022:22 \
  --name gitlab \
  --restart always \
  --volume $GITLAB_HOME/config:/etc/gitlab \
  --volume $GITLAB_HOME/logs:/var/log/gitlab \
  --volume $GITLAB_HOME/data:/var/opt/gitlab \
  --shm-size 256m \
  --env GITLAB_LOG_LEVEL=warn \
  --env GITLAB_ROOT_PASSWORD=secret \
  --env GITLAB_HOST=http://gitlab.shire.lan \