permit overriding of DOCKERHUB_USER
permit overriding of DOCKERHUB_USER

which in turn enables operation from private registries

Closes #887.

aaam/weave,bboreham/weave,OnePaaS/weave,paulbellamy/weave,bboreham/weave,BWITS/weave,squaremo/weave,cgvarela/weave,dpw/weave,narry/weave,errordeveloper/weave,squaremo/weave,dpw/weave,2opremio/weave,kidaa/weave,OnePaaS/weave,rade/weave,kidaa/weave,n054/weave,awh/weave,beni55/weave,paladin74/weave,philipz/weave,BWITS/weave,philipz/weave,n054/weave,paladin74/weave,2opremio/weave,beni55/weave,weaveworks/weave,weaveworks/weave,bboreham/weave,awanke/weave,squaremo/weave,paulbellamy/weave,aaam/weave,kidaa/weave,BWITS/weave,ycaihua/weave,philipz/weave,narry/weave,aaam/weave,dpw/weave,awh/weave,2opremio/weave,philipz/weave,cgvarela/weave,cgvarela/weave,achanda/weave,awanke/weave,awanke/weave,achanda/weave,kidaa/weave,beni55/weave,squaremo/weave,OnePaaS/weave,awh/weave,tomwilkie/weave,achanda/weave,bboreham/weave,errordeveloper/weave,dpw/weave,kidaa/weave,tomwilkie/weave,achanda/weave,bboreham/weave,ycaihua/weave,tomwilkie/weave,rade/weave,paladin74/weave,weaveworks/weave,awanke/weave,rade/weave,n054/weave,awh/weave,philipz/weave,aaam/weave,weaveworks/weave,awh/weave,n054/weave,dpw/weave,n054/weave,paladin74/weave,squaremo/weave,BWITS/weave,2opremio/weave,cgvarela/weave,ycaihua/weave,paulbellamy/weave,tomwilkie/weave,OnePaaS/weave,paulbellamy/weave,errordeveloper/weave,rade/weave,ycaihua/weave,2opremio/weave,beni55/weave,paladin74/weave,aaam/weave,awanke/weave,rade/weave,ycaihua/weave,narry/weave,beni55/weave,narry/weave,cgvarela/weave,OnePaaS/weave,BWITS/weave,errordeveloper/weave,errordeveloper/weave,tomwilkie/weave,narry/weave,achanda/weave