text: Title: Launching docker container in interactive mode
  Body: I was expecting `repo2docker https://github.com/binder-examples/conda-freeze /bin/bash` to give me a shell inside the container. Instead it builds the image and then exits straight away.
  
  I think this is because we would have to do the equivalent of `docker run -it <image> /bin/bash` but don't. Should we add a `--interactive` CLI flag that turns on this behaviour?
  
  **Note:** a solution to this is outlined in https://github.com/jupyter/repo2docker/issues/599#issuecomment-483402517. This would be a good way to get involved with contributing to repo2docker and something to help out with in general.