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).

Uhm, docker is just a wrapper around linux's kernel features, plus image format. Image format being just bunch of tarballs that needs to be mounted by _something_ layer by layer.

You can write it any language that runs on OS with linux's kernel. There are similar things written and damn bash. Does that make bash a system language?

The original version of Docker was written in Python, and the original version of Kubernetes was written in Java, so your argument doesn't hold.

Would this be the part of my argument that doesn't mention Python (which I personally consider a systems language) at all? Or the part of my argument where I say, "you certainly could use them"?

It just means that golang isn't anything special when it comes to writing tools like these, and is in the same ballpark as python and Java (it's somewhere in between the two of them, better than python but worse than Java), and you'd have to use the same escape hatches in golang as them to implement those tools.