Hardcoded command "/bin/sh -c" breaks containers with entrypoints

[parente]

The command prefix here:

https://github.com/jupyter/tmpnb/blob/master/dockworker.py#L112

causes problems when container images have custom entrypoints like the one here:

https://github.com/jupyter/docker-stacks/blob/master/minimal-kernel/Dockerfile#L46

Opening this for reference since I routinely forget about this problem. Maybe there's a fix we can do in tmpnb to support the current default plus the custom entrypoint. Or maybe tmpnb README puts a stake in the ground about how the container entrypoint needs to be crafted.

[rgbkrk]

Ah and that one was put it in to make it easier to launch the current notebooks. I'm ok with overriding this.