code: def checkDockerImageExists(appliance):
      
      if currentCommit in appliance:
          return appliance
      registryName, imageName, tag = parseDockerAppliance(appliance)
  
      if registryName == 'docker.io':
          return requestCheckDockerIo(origAppliance=appliance, imageName=imageName, tag=tag)
      else:
          return requestCheckRegularDocker(origAppliance=appliance, registryName=registryName, imageName=imageName, tag=tag)
docs: Attempts to check a url registryName for the existence of a docker image with a given tag.
  
      :param str appliance: The url of a docker image's registry (with a tag) of the form:
                            'quay.io/<repo_path>:<tag>' or '<repo_path>:<tag>'.
                            Examples: 'quay.io/ucsc_cgl/toil:latest', 'ubuntu:latest', or
                            'broadinstitute/genomes-in-the-cloud:2.0.0'.
      :return: Raises an exception if the docker image cannot be found or is invalid.  Otherwise, it
               will return the appliance string.
      :rtype: str