I've been playing around with the new Docker beta app for Mac and its been great fun but the one issue I've noticed is that my Time Machine backups are massive all of a sudden. After some digging around with BackupLoupe the cause of this is Docker.
Using BackupLoupe I could see that this is a constant issue:
If you don't want to install anything you can also run the following from Terminal.app
$ du -hsc ~/Library/Containers/com.docker.docker/
And you should see something like
5.8G ~/Library/Containers/com.docker.docker/
Simple fix, Docker is just the thin front-end to manage my instances so for now ~/Library/Containers/com.docker.docker/ is excluded.