nixos/docker: Include ZFS commands in PATH for ZFS storagedriver

When using the ZFS storagedriver in docker, it shells out for the ZFS
commands. The path configuration for the systemd task does not include
ZFS, so if the driver is set to ZFS, add ZFS utilities to the PATH.

This will resolve https://github.com/NixOS/nixpkgs/issues/10127

[Bjørn: prefix commit message with "nixos/docker:", remove extra space
before ';']