Expand the documentation about docker images startup command

[consideRatio]

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I made a suggested edit in https://github.com/jupyterhub/zero-to-jupyterhub-k8s/pull/2449#discussion_r733750167 to the https://zero-to-jupyterhub.readthedocs.io/en/latest/jupyterhub/customizing/user-environment.html documentation. Instead of trying to resolve that there and then, I want to go for a merge and tackle this later.