Use ephemeral storage for disk mutations

[viveklak]

Fixes #96. Prerequisite for #78 and should help with #104 as well.

Two changes here:

1. Set the global config and cache directories to be in /tmp. The Docker image by default seemed to be using /root/.xyz which is the reason for Typescript programs using external dependencies do not run #96.
2. We should be avoiding writing state back into the image (overlay). /tmp is now an emptyDir mount which will make the entire /tmp directory ephimeral for the lifecycle of the pod. Once the pod dies, the storage is immediately released. This would be valuable for Move the reconciliation loop to a model where each run executes as a k8s Job #78

[lukehoban]

LGTM