rkt: fix failing TestRktDriver_UserGroup test
rkt: fix failing TestRktDriver_UserGroup test

Started failing due to the docker redis image switching from Debian
jessie to stretch:
https://github.com/docker-library/redis/commit/53f86805506b103b503fd392e029929290fe5346#diff-acff46b161a3b7d6ed01ba79a032acc9

Switched from Debian based image to Alpine to get a working `ps` command
again (albeit busybox's stripped down implementation)

burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,nak3/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,nak3/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,nak3/nomad,dvusboy/nomad,nak3/nomad,nak3/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad