I started receiving notifications that my docker.img file on Unraid was filling up. After looking into it some, I found that the MQTT container was logging crazy amounts of data (15+GB). Turns out there is a parameter that you can add to limit the log size (for Docker containers). I added this to the docker configuration:
--log-opt max-size=50m --log-opt max-file=1
then restarted and bam! Back to a reasonable size.
The post with this info is here: Docker FAQ