Investigate GCS reader panic on response header

Panic caught via Sentry https://new-sentry.gitlab.net/organizations/gitlab/issues/1327478/?alert_rule_id=23&alert_type=issue&notification_uuid=efcd584f-e9eb-4601-b869-9d73ed75099c&project=13

github.com/docker/distribution/registry/storage/driver/gcs.(*driver).Reader(0xc0007ef570, {0x25a1ab8, 0xc0006c2930}, {0xc001f51b20, 0x69}, 0x0)
	/go/src/github.com/docker/distribution/registry/storage/driver/gcs/gcs.go:378 +0x11f

The line of code is https://gitlab.com/gitlab-org/container-registry/-/blob/master/registry/storage/driver/gcs/gcs.go#L378

	if res.Header.Get("Content-Type") == uploadSessionContentType { // FAILED HERE
		defer res.Body.Close()
		return nil, storagedriver.PathNotFoundError{Path: path, DriverName: driverName}
	}

It might be possible that either the res or res.Header is nil.