Docker didn't have much of a choice. At the time they had two options: C/C++ or Go (Rust wasn't usable at). They went with the right choice at the time, Go. Unfortunately nobody really knew about all of the pitfalls of Go (there are many upsides, but writing a container runtime in Go is definitely not one of them).

Right now, I would definitely recommend people write things like this in Rust. In fact I was planning on working on something similar. :P