The terms task directory and allocation directory are used throughout the
documentation but these directories are not the same as the `NOMAD_TASK_DIR`
and `NOMAD_ALLOC_DIR` locations. This is confusing when trying to use the
`template` and `artifact` stanzas, especially when trying to use a destination
outside the Nomad-mounted directories for Docker and similar drivers.
This changeset introduces "allocation working directory" to mean the location
on disk where the various directories and artifacts are staged, and "task
working directory" for the task. Clarify how specific task drivers interact
with the task working directory.
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