Do not fail when no docker registry auth is available
Do not fail when no docker registry auth is available

this amends the behaviour introduced with #2651
and allows pulling public images when docker.auth.helper is set

frundh/nomad,frundh/nomad,kmalec/nomad,dvusboy/nomad,frundh/nomad,dvusboy/nomad,kmalec/nomad,frundh/nomad,nak3/nomad,hashicorp/nomad,burdandrei/nomad,kmalec/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,Ashald/nomad,nak3/nomad,nak3/nomad,burdandrei/nomad,kmalec/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,frundh/nomad,nak3/nomad,frundh/nomad,dvusboy/nomad,Ashald/nomad,kmalec/nomad,dvusboy/nomad,burdandrei/nomad,kmalec/nomad,nak3/nomad,burdandrei/nomad,Ashald/nomad,Ashald/nomad,hashicorp/nomad,frundh/nomad,Ashald/nomad,hashicorp/nomad,nak3/nomad,kmalec/nomad,burdandrei/nomad,nak3/nomad,Ashald/nomad,Ashald/nomad