Harmonize / across all name-related commands/Validate images names

Docker-DCO-1.1-Signed-off-by: Djibril Koné <kone.djibril@gmail.com> (github: enokd)

docs/registry_test.go

@@ -206,4 +206,8 @@ func TestValidRepositoryName(t *testing.T) {
 		t.Log("Repository name should be invalid")
 		t.Fail()
 	}
+	if err := validateRepositoryName("docker///docker"); err == nil {
+		t.Log("Repository name should be invalid")
+		t.Fail()
+	}
 }