If you want to run a game server, you mostly will do not need to rebuild this image.
You'll want to write run.sh script which will be executed after updating steamcmd (in /data/steamcmd directory). The script should be mounted as a volume. /data can be also a volume.
Docker registry: https://hub.docker.com/r/pyyy/steamcmd/
Based on Ubuntu 16.04. Libraries required to start SteamCMD and Don't Starve server are installed.