I came here looking for the same thing.   Running in to exactly the same issue with the public test beta.  We have 1-6 players at any given time.  We've used the ich777 container several times before without issue.  The only difference with this one is that I'm using a different appdata location and I've added:   '-beta public-test -betapassword yesimadebackups'   to the 'GAME_ID'.   One interesting thing that I've noticed is that world files are NOT being