old_contents: """
  Migrate config to docker hosts list
  """
  import os
  import yaml
  
  
  filename = os.path.join('data', 'configs.yaml')
  with open(filename) as handle:
      data = yaml.load(handle)
  
  host = data.pop('docker_host')
  data['docker_hosts'] = [host]
  
  with open(filename, 'w') as handle:
      yaml.dump(data, handle, default_flow_style=False)
  
new_contents: """
  Migrate config to docker hosts list
  """
  import os
  import sys
  import yaml
  
  
  filename = os.path.join('data', 'configs.yaml')
  try:
      with open(filename) as handle:
          data = yaml.load(handle)
  except FileNotFoundError:
      # This is fine; will fail for new installs
      sys.exit(0)
  
  host = data.pop('docker_host')
  data['docker_hosts'] = [host]
  
  with open(filename, 'w') as handle:
      yaml.dump(data, handle, default_flow_style=False)
  
subject: Handle no config.yaml in migrations