[FEATURE] PV support

[tetchel]

One of the major value-adds of SH runners is the persistent disk, so setup steps (eg installing tooling, runtimes) and some build steps that can benefit from caching (eg building container images) can be sped up.

Right now, the disk is lost when the pod terminates.

A user should be able to bind a persistent volume to the runner pod so that disk can be re-used by subsequent pods (if desired).