code: def detect_registry_url(client, auto_login=True):
      
      repo = client.repo
      config = repo.config_reader()
  
      # Find registry URL in .git/config
      remote_url = None
      try:
          registry_url = config.get_value('renku', 'registry', None)
      except NoSectionError:
          registry_url = None
  
      remote_branch = repo.head.reference.tracking_branch()
  
      if remote_branch is not None:
          remote_name = remote_branch.remote_name
          config_section = 'renku "{remote_name}"'.format(
              remote_name=remote_name
          )
          try:
              registry_url = config.get_value(
                  config_section, 'registry', registry_url
              )
          except NoSectionError:
              pass
          remote_url = repo.remotes[remote_name].url
  
      if registry_url:
          # Look in [renku] and [renku "{remote_name}"] for registry_url key.
          url = GitURL.parse(registry_url)
      elif remote_url:
          # Use URL based on remote configuration.
          url = GitURL.parse(remote_url)
  
          # Replace gitlab. with registry. unless running on gitlab.com.
          hostname_parts = url.hostname.split('.')
          if len(hostname_parts) > 2 and hostname_parts[0] == 'gitlab':
              hostname_parts = hostname_parts[1:]
          hostname = '.'.join(['registry'] + hostname_parts)
          url = attr.evolve(url, hostname=hostname)
      else:
          raise errors.ConfigurationError(
              'Configure renku.repository_url or Git remote.'
          )
  
      if auto_login and url.username and url.password:
          try:
              subprocess.run([
                  'docker',
                  'login',
                  url.hostname,
                  '-u',
                  url.username,
                  '--password-stdin',
              ],
                             check=True,
                             input=url.password.encode('utf-8'))
          except subprocess.CalledProcessError:
              raise errors.AuthenticationError(
                  'Check configuration of password or token in the registry URL'
              )
  
      return url
docs: Return a URL of the Docker registry.