text: Title: Command to run already built image
  Body: Often, I would like to play with what volumes are mounted, or what command is executed, without having to rebuild the docker image.
  
  This takes a long time when working on a JupyterLab extension that depends on JupyterLab master and so has to rebuild all of JupyterLab every image build.
  
  Currently, I am looking for the image name manually with `docker image ls` and running it manually. This isn't ideal, because then it isn't run in the same way as `repo2docker` is running it. You pass some extra options on start, like a port and the custom display URL. I would like to keep all these nicetities and not have to hunt in the image list for the most recent one.
  
  I thought this is what the `--no-build` argument would do, but I guess it not only doesn't build but doesn't start. For example, [`docker-compose up`](https://docs.docker.com/compose/reference/up/) has a `--no-build` argument that won't build the images, but will start up the services. 