CI : Use GitHub docker container registry
CI : Use GitHub docker container registry

This is preferable to managing a bunch of separate accounts for DockerHub.

hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,hradec/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,hradec/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer