Specify command and extra files in project.clj

Allows config to be given in project.clj with the form :

:uberimage {:cmd ["/bin/dash" "/myrunscript" "param1" "param2"]
            :files {"myrunscript" "docker/myrunscript"}}

- The :cmd value maps directly to a Dockerfile CMD statement

- The :files value is a map of additional files to be copied into the
  docker image. Keys are docker image target paths and values are lein
  project source paths

This permits more flexibility in starting the containerized program,
allowing things such as configuring extra environment with a script.