Run containers as hashr user to remove need for "sudo rm"

Currently when cleaning up the temp files we use sudo: https://github.com/google/hashr/blob/8c20ba5d128879cf5e317b43ecc3212bf572fb74/core/hashr/hashr.go#L235 I presume this is because some of the importers run a docker image (as root) which means the produced files are owned by root. If we run that container as the same user hashr is running as they will be owned by that user and we don't need to do "sudo rm ..." which could be dangerous if there is a bug.