libcontainer based drivers (e.g. exec, java) should default to running
processes as `nobody` [1]; but libcontainer treats empty user as `root`
in our case (either because of default or due to `root` being current
user).

[1] https://github.com/hashicorp/nomad/blob/94c28a4c6cc45f1b377d50a6bff2017b46c43d18/website/source/docs/job-specification/task.html.md#task-parameters