public CredentialRetriever legacyDockerConfig(Path dockerConfigFile) {
    return dockerConfig(
        DockerConfigCredentialRetriever.createForLegacyFormat(
            imageReference.getRegistry(), dockerConfigFile));
  }
Creates a new {@link CredentialRetriever} that tries to retrieve credentials from a legacy
Docker config file.

@param dockerConfigFile the path to a legacy docker configuration file
@return a new {@link CredentialRetriever}
@see DockerConfigCredentialRetriever
jib-core/src/main/java/com/google/cloud/tools/jib/frontend/CredentialRetrieverFactory.java