This is becoming really really annoying.
I think at the very least you should expose a config setting where one can set the directory location to another disk.
And even better would be if the big images files were actually hosted and visible on the Mac, so one can manually remove a file or two when the docker machine doesn’t start up again. So basically mount the files into the docker machine like a shared file system.
If that is technically not possible, then mount the directory where the big files are on the docker machine to a directory on the mac host, so if and when the docker starts up again, one can go and remove files that are no longer required, without having to throw away the whole gcow2 file and start all downloads all over again.