include steam and beginnings of arma3 and arma3-epoch server

todo: 

[ ] test arma3 dockerfile
  [ ] test assumptions about `wget steamcmd | tar`
  [ ] configure arma3 to pull persistant data from non `/opt/*` paths
[ ] create arma3-data dockerfile ?
[ ] wait for arma3 epoch server files to be released and complete arma3 epoch dockerfile