builder/copy-unix: fix filepath.Walk args
builder/copy-unix: fix filepath.Walk args

Mark unused WalkFunc arguments as such, to get rid of the
following linterwarning:

> builder/dockerfile/copy_unix.go:28:71: SA4009: argument err is overwritten before first use (staticcheck)
> 	return filepath.Walk(source, func(fullpath string, info os.FileInfo, err error) error {

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

mahak/docker,pradipd/moby,mahak/docker,djs55/docker,cpuguy83/docker,thaJeztah/docker,jimjag/docker,moul/docker,paralin/docker,jimjag/docker,yosifkit/docker,mahak/docker,SvenDowideit/docker,nalind/docker,Ramzec/docker,yosifkit/docker,pradipd/moby,nawawi/docker,konstruktoid/docker-1,ibuildthecloud/docker,kojiromike/docker,konstruktoid/docker-1,jvanz/docker,ibuildthecloud/docker,nalind/docker,yosifkit/docker,Ramzec/docker,ibuildthecloud/docker,AkihiroSuda/docker,Mokolea/docker,SvenDowideit/docker,dims/docker,gnailuy/docker,thaJeztah/docker,moby/moby,AkihiroSuda/docker,Ramzec/docker,justincormack/docker,AkihiroSuda/docker,cpuguy83/docker,konstruktoid/docker-upstream,jimjag/docker,thaJeztah/docker,docker/docker,moby/moby,jvanz/docker,yongtang/docker,flavio/docker,bearstech/moby,jameszhan/docker,jthelin/docker,vdemeester/docker,moul/docker,trebonian/docker,bearstech/moby,yosifkit/docker,gnailuy/docker,justincormack/docker,yongtang/docker,konstruktoid/docker-1,kojiromike/docker,gnailuy/docker,trebonian/docker,djs55/docker,Mokolea/docker,djs55/docker,estesp/docker,dims/docker,vdemeester/docker,ibuildthecloud/docker,Mokolea/docker,SvenDowideit/docker,nawawi/docker,moby/moby,jameszhan/docker,bearstech/moby,thaJeztah/docker,Ramzec/docker,AkihiroSuda/docker,konstruktoid/docker-upstream,flavio/docker,yongtang/docker,jameszhan/docker,dims/docker,nawawi/docker,jvanz/docker,konstruktoid/docker-upstream,paralin/docker,nalind/docker,kojiromike/docker,flavio/docker,paralin/docker,docker/docker,trebonian/docker,jimjag/docker,cpuguy83/docker,nalind/docker,mahak/docker,estesp/docker,trebonian/docker,moby/moby,docker/docker,estesp/docker,moul/docker,paralin/docker,cpuguy83/docker,vdemeester/docker,gnailuy/docker,justincormack/docker,Mokolea/docker,bearstech/moby,flavio/docker,jthelin/docker,moul/docker,konstruktoid/docker-upstream,ibuildthecloud/docker,nawawi/docker,pradipd/moby