Subsequent shells e.g. `kubectl/docker exec` don't enter as the expected user

I.e. using https://github.com/coder/envbuilder-starter-devcontainer, which sets up user `coder` when `docker exec` get:

`root@remote-development:/workspaces/envbuilder-starter-devcontainer#`

Can then `sudo -su coder` and the environment is as expected.

Reproduce the behavior with the envbuilder-starter-devcontainer example and a subsequent docker exec or kubectl exec shell. Investigate how the container user is selected, then verify that subsequent shells enter as coder rather than root and retain the expected environment.

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