Improve documentation on how singleuser images behave when hub is restarted

[manics]

The default is for spawned images not to be deleted when they're stopped. This can have unexpected effects if the JupyterHub configuration or deployment is changed since the restarted container keeps it's old environment variables and command-line. This means some (is there a list???) of the connection parameters are re-used, but they may be out of date.

See https://discourse.jupyter.org/t/why-does-jupyterhub-not-see-the-docker-network-i-have-created/5275/

Is there a good way to handle this, especially as it's more likely to hit people new to JupyterHub since they're most likely to be playing with their configuration?

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[1kastner]

For me using docker-compose (up/down) and manually deleting docker volumes served the purpose. Maybe that could be a recommendation?

[minrk]

I think a great enhancement would be when a stopped container is about to resume, go through and check for fields that differ from what we would be starting and warn about the differences, and possibly suggesting that the container be removed. We could also have a secondary flag to delete containers that don't match rather than just warning about a likely impending failure.