Commit incomplete - how to safely free up disk space

A docker commit was accidentally started with a very large diff.
The commit was then interrupted by stopping the service (obviously not the best way…)
It looks as if it the disk space it still taken up by the partial commit, and it’s not clear how to free it up.
Various “docker prune” methods on images and volumes have not helped…
Any tips on how to safely free up this space?

Ubuntu 16.04
Docker 17.09 CE
Running on an x86 AWS instance