code: func (o *OsEnvLookup) Lookup(key string, config *config.ServiceConfig) []string {
  	ret := os.Getenv(key)
  	if ret == "" {
  		return []string{}
  	}
  	return []string{fmt.Sprintf("%s=%s", key, ret)}
  }
docs: // Lookup creates a string slice of string containing a "docker-friendly" environment string
  // in the form of 'key=value'. It gets environment values using os.Getenv.
  // If the os environment variable does not exists, the slice is empty. serviceName and config
  // are not used at all in this implementation.