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
so many assumptions!