From what I can tell, docker has some leakage here. Temp files are not removed correctly, resulting in the build up of disk storage usage.
See the following for more details
https://stackoverflow.com/questions/46672001/is-it-safe-to-clean-docker-overlay2
https://forums.docker.com/t/some-way-to-clean-up-identify-contents-of-var-lib-docker-overlay/30604
https://docs.docker.com/storage/storagedriver/overlayfs-driver/

Im going to write a clean up script and add that to the cron. I dont believe there is an easy way around this issue as docker trades off disk storage for simplicity