openssh-server doesn't start in Docker container

Container vs. Image

The RUN statement is used to run commands when building the docker image.
With ENTRYPOINT and CMD you can define what to run when you start a container using that image.
See Dockerfile Reference for explanations how to use them.

Services

There is not preinstalled init-system in the containers, so you cannot use service ... start in a container.
Consider starting the process in the CMD statement as foreground process or use an init-system like Phusion or Supervisord.