Specify user in nomad alloc exec

[davidlublink]

Proposal

docker exec -it -u root abc123 bash

The above docker command allows you to override any 'USER' command specified in the Dockerfile and run bash as root.

There does not seem to be any way to override this user with 'nomad alloc exec'.

Use-cases

This is a useful feature when debugging/troubleshooting some allocations where the operator needs to act as root for a given allocation.

Attempted Solutions

So an operator can work around this limitation by logging into the Nomad client that is running the allocation and from there they can docker exec -it -u root cba321 bash

[tgross]

Without digging in too much, I suspect the reason this doesn't already exist is because in non-Docker task drivers we've already had a chance to guard the user selected from the host via controls like client.user.denylist and Sentinel policies. And then in image-isolation tasks like Docker we have little ability to guard which users are acceptable (but also care a lot less). So we'd probably need to wire up all the same set of controls to the Allocation API in order to do so. Still, seems like a reasonable idea. I'll mark it for further roadmapping.