Migrate the loaders from the tmpfs workaround to a real in memory emptydir

the emptyDir can be be backed by in-memory storage: https://kubernetes.io/docs/concepts/storage/volumes/#emptydir

IIRC, the feature was not available when we started deploying on kube so we used the local-storage using a tmpfs as a workaround.

The result is the space used by a local-persistent volume is not visible in the pod properties and not easily tunable.

Using this kind of in-memory ephemeral volumes will allow to manage more precisely the space allocation of the loaders and more importantly to have a clear vision of the resource consumption of the pods (it's considered as memory consumption).