Depends what you mean by "systems language" (many common definitions turn out to be equivalent to "drop-in replacement for the platform language," which makes the argument circular), but the choice of Go as an implementation language for Docker and Kubernetes puts it pretty firmly in the "systems language" space IMO. Container management requires more systems-level functionality than C# and Java are really geared towards (although you certainly could use them, perhaps with a tablespoon of FFI).