Improve the "how to prepare your Dockerfile" section

[choldgraf]

Hit three main use-cases:

• Option 1: Use jupyter-dockerstacks as a pre-existing image
• Option 2: Creating your own Dockerfile from scratch
• Option 3: Use repo2docker

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Current in 0.7.0:

• Selection of jupyter/docker-stacks
• Extension of jupyter/docker-stacks

[consideRatio]

I restructured this section for jupytercon2018, many started using repo2docker and failing since for different reasons and unmet expectations on the outcome.

The current docs have:

1. Choosing an image from docker-stacks
2. Extending such image

I'm open for the update of:
3. Short note about building one from scratch and known but perhaps not exhaustive requirements on the Dockerfile. (jupyterhub package, the startup command, uid, more?)

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This means that we currently no longer provide a repo2docker suggested path to get a suitable z2jh image. I found that it was a lot more complicated and led to complications with storage etc as compared to the docker-stack options. This led to the opinion that repo2docker shouldn't be suggested as an option within z2jh to build user images.

[manics]

Is this still on the todo list?

[consideRatio]

Yepp I think so =/

[consideRatio]

I'm closing this as we have "Choose and use an existing Docker image" and "Customize an existing Docker image", I feel this is sufficient for now without explicit user requests for more. See: https://zero-to-jupyterhub.readthedocs.io/en/latest/customizing/user-environment.html