docker.go: do not concatenate url in ping
docker.go: do not concatenate url in ping

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

mikebrow/skopeo,mikebrow/skopeo,mtrmac/skopeo,projectatomic/skopeo,containers/image,jwhonce/skopeo,projectatomic/skopeo,mtrmac/skopeo,jwhonce/skopeo,masters-of-cats/image,mtrmac/skopeo,containers/image