Feature Request: Per-task docker auth helper

As per the docs, it looks like the auth helper definition is only available as a global per repo setting https://www.nomadproject.io/docs/drivers/docker.html#plugin_auth_helper

It would be nice to be able to have the option to include this directly in the task definition. For my use case, I am trying to generate restricted short term credentials based on the specific repo being accessed.

Based on the code here https://github.com/hashicorp/nomad/blob/master/drivers/docker/driver.go it looks like this would be a relatively easy addition.

Start with the Docker driver implementation in drivers/docker/driver.go and the plugin auth helper documentation. Trace how the repository-level helper is configured and how task definitions are processed. Done means a task can specify its own helper for repository-specific credentials while the existing global setting remains supported; add or update focused tests for both paths if the repository provides them.

Written by the indexing model from the issue text.