code: func (l *ComposableEnvLookup) Lookup(key string, config *config.ServiceConfig) []string {
  	result := []string{}
  	for _, lookup := range l.Lookups {
  		env := lookup.Lookup(key, config)
  		if len(env) == 1 {
  			result = env
  		}
  	}
  	return result
  }
docs: // Lookup creates a string slice of string containing a "docker-friendly" environment string
  // in the form of 'key=value'. It loop through the lookups and returns the latest value if
  // more than one lookup return a result.