remove utils.go

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
move opts out of pkg because it's related to docker

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #4562 from vieux/remove_utils_in_docker_package

Remove utils in docker package