Feature request: allow administrator to restrict host volume mappings

As an administrator, I want to be able to whitelist -- or, alternatively blacklist -- host volumes that can be mounted into a task container. This would mitigate certain kinds of host data leakage vulnerabilities (e.g., mounting `/etc/shadow` into a container) that could be exploited by allowing tasks to run with arbitrary and unfiltered volume mount specifications.

No files, tests, or entry points are named. Start by locating the task-container volume-mount configuration and determine whether whitelist or blacklist semantics are intended; done means administrators can enforce the selected policy and blocked mappings cannot be used.

Written by the indexing model from the issue text.