Support for '--user' option

[onlyjob]

Please implement support for user option: https://www.nomadproject.io/docs/job-specification/task.html#user

This is useful to select user for running a command inside container (e.g. podman run --user=nobody --init --rm docker.io/library/debian:stable whoami).

Thanks.

[onlyjob]

Docker driver supports that: https://www.nomadproject.io/docs/drivers/docker.html#pids_limit:

Additionally, the docker driver supports customization of the container's user through the task's user option.

[towe75]

See latest commit. It includes the feature, a readme snipped and a unit test. Please check.