String getServerUrl() {
    return serverUrl;
  }
New properties.

@param serverUrl the server URL for the registry (for example, {@code gcr.io})
@param imageName the image/repository name (also known as, namespace)
@param sourceImageName additional source image to request pull permission from the registry
jib-core/src/main/java/com/google/cloud/tools/jib/registry/RegistryEndpointRequestProperties.java