Add volumes requirement

[jdidion]

The disks requirement is intended for mounting empty, read/writable volumes for use by the task. However, there are cases where a task might need access to an existing volume, with e.g. reference genomes.

We should add a volumes requirement that takes a volume spec (string) or list of volume specs and causes those volumes to be mounted read-only in the host environment.

A volume spec is either a path, in which case that path must exist on the host system and is mounted at the same path in the container, or a pair of paths, in which case the first path is the host system path and the second path is the mount point within the container.