Docker is about deployment. The isolation model is built on the same tooling that systemd uses, but the usage model is all about how to come up with an easily isolated runtime environment that can be published and extended in a big development community.

Basically systemd has nothing like a dockerfile, that's not what it's for.

I think there's a good argument though that Docker should be retargetting its encapsulation work to a lower level OS primitive where possible (which would be systemd on linux) instead of rolling its own.