code: @VisibleForTesting
    RegistryAuth anyRegistryAuth(final Path configPath) throws IOException {
      final ImmutableCollection<RegistryAuth> registryAuths =
          authForAllRegistries(configPath).configs().values();
      return registryAuths.isEmpty()
          ? RegistryAuth.builder().build()
          : registryAuths.iterator().next();
    }
docs: Return a single RegistryAuth from the config file.
  If there are multiple RegistryAuth entries, which entry is returned from this method
  depends on hashing and should not be considered reliable.
  If there is only one entry, however, that will be the one returned. This is the
  primary use of this method, as a useful way to extract a RegistryAuth during testing.
  In that environment, the contents of the config file are known and controlled. In a
  production environment, the contents of the config file are less predictable.
  
  @param configPath Path to the docker config file.
  @return Some registry auth value.