OK, got it. I upgraded the docker version on my machine and now :
server_1 | 2018/06/19 15:14:13 groups.go:356: Serving tablet for: _predicate_
server_1 | 2018/06/19 15:14:13 mutation.go:158: Done schema update predicate:"_predicate_" value_type:STRING list:true
Looks like I’m good to go with the rest of the tutorial. Thanks for your help and telling me something was wrong, it helped me investigate on docker itself.
Phew.