I've tried this and it is not that easy, docker containers are meant to run one and only one process. So, for instance my puppet started an upstart job, and this crashes the docker build process.
The Dockerfile shouldn't run any processes that need to persist between 2 stages as each stage will be created in a new instance. You can either do fancy one-liner bash scripts (my favorite) to configure simple service / app startup scripts, or include a configuration file from elsewhere using the `ADD` directive. The only non ephemeral command should be the final `CMD` or `ENTRYPOINT` as far as I can work out.
I too am just learning this stuff, but that should hopefully help you out!
