Expose RCON port

[mmonkey]

Dockerfile isn't exposing the RCON port.

Could do something like:

EXPOSE ${GAME_PORT} ${RCON_PORT}

[KagurazakaNyaa]

EXPOSE in Dockerfile actually only has a prompting effect. The effective parameters are the parameters in docker run and the configuration in docker-compose.yml. https://docs.docker.com/engine/reference/builder/#expose

I will modify the Dockerfile and sample docker-compose.yml for reference.