Add initial_user traitlet to change user that container is started with

[matthewwiese]

This PR adds the initial_user traitlet, which may be used to change the user that the spawned container is started with.

Per my comment here I was encountering a mounted directory with root permissions that meant the default jovyan user had no access.

In order to follow the suggestion here, I needed DockerSpawner to start the container as root so that the permissions may be changed. This can be done via this traitlet like so:

c.DockerSpawner.initial_user = "root"

This PR is inspired by this comment, which I saw was not yet implemented. It is useful beyond my specific case, such as when changing user-related configuration in a Jupyter Docker Stack

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[matthewwiese]

I have discovered that this can just be done via the existing extra_create_kwargs:

c.DockerSpawner.extra_create_kwargs = {
    "user": "root"
}

I can close this PR as it's superfluous given the above knowledge.

Related: tests/test_dockerspawner.py::test_post_start fails for this PR because there is no jovyan user in the busybox:1.29.1 image.

[manics]

I think extra_create_kwargs is a good solution, especially since if you add a user config it should also allow a numeric UID, and it should default to the containers default.

Do you think it'd be useful to add this as an example in the documentation?

[matthewwiese]

Do you think it'd be useful to add this as an example in the documentation?

Yeah, I think an example in the docs would be handy, if only to increase the visibility of extra_create_kwargs. When I was investigating the problem that lead to this PR, I had read the entry for extra_create_kwargs in the API reference but it didn't immediately dawn on me how it may be useful.