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 ';']
This version of module has disabled socketActivation, because until
nixos upgrade systemd to at least 214, systemd does not support
SocketGroup. So socket is created with "root" group when
socketActivation enabled. Should be fixed as soon as systemd upgraded.
Includes changes from #3015 and supersedes #3028