The Dockerfile will build an image for running SteamCMD like an executable.
An simple example that will run SteamCMD and login as an anonymous user:
docker run -it kmallea/steamcmd +login anonymous
A more useful example that will login anonymously and download CS:GO to run as a dedicated server. Again, just pass args since it works like an executable:
docker run -it kmallea/steamcmd \ +login anonymous \ +force_install_dir ../csgo \ +app_update 740 validate \ +quit