So I have been running my current windows installation for a couple of years, and was starting to run out of disk space on the C-drive as one does after a while.
After a quick inspection with WinDirStat, it was clear that all my space was taken up by ext4.vhdx
As indicated by the path it is used by docker for saving its data.
To get a better understanding how the data is used, run
docker system df
To reclaim space from the build cache you can use
docker builder prune --filter until=24h
This way you can keep the most recent cache. If you just want to do a deep cleaning it is also possible to use
docker system prune -a
However I did that and it failed to clean the build cache, although it claims it should do that.
This by itself down shrink the size of the vhdx file. To do so you must shutdown WSL
wsl --shutdown and run a the following powershell command
optimize-vhd -Path .\ext4.vhdx -Mode full