code: def requestCheckRegularDocker(origAppliance, registryName, imageName, tag):
      
      ioURL = 'https://{webhost}/v2/{pathName}/manifests/{tag}' \
                ''.format(webhost=registryName, pathName=imageName, tag=tag)
      response = requests.head(ioURL)
      if not response.ok:
          raise ApplianceImageNotFound(origAppliance, ioURL, response.status_code)
      else:
          return origAppliance
docs: Checks to see if an image exists using the requests library.
  
      URL is based on the docker v2 schema described here:
      https://docs.docker.com/registry/spec/manifest-v2-2/
  
      This has the following format:
      https://{websitehostname}.io/v2/{repo}/manifests/{tag}
  
      Does not work with the official (docker.io) site, because they require an OAuth token, so a
      separate check is done for docker.io images.
  
      :param str origAppliance: The full url of the docker image originally
                                specified by the user (or the default).
                                e.g. "quay.io/ucsc_cgl/toil:latest"
      :param str registryName: The url of a docker image's registry.  e.g. "quay.io"
      :param str imageName: The image, including path and excluding the tag. e.g. "ucsc_cgl/toil"
      :param str tag: The tag used at that docker image's registry.  e.g. "latest"
      :return: Return True if match found.  Raise otherwise.