OK I found the problem!
When I first installed discourse, I created an ‘import’ container to import messages from groups.io. I removed this container and deleted the yml files from the containers folder, and it has been running fine (with a number of reboots) since.
Something in the docker upgrade appears to have re-activated it, I ran docker container list and there was a container called import!
I deleted it from docker and restarted the app container and all seems to be well.
Thanks
Phil