code: func (c *imageProber) Probe(parentID string, runConfig *container.Config) (string, error) {
  	if c.cacheBusted {
  		return "", nil
  	}
  	cacheID, err := c.cache.GetCache(parentID, runConfig)
  	if err != nil {
  		return "", err
  	}
  	if len(cacheID) == 0 {
  		logrus.Debugf("[BUILDER] Cache miss: %s", runConfig.Cmd)
  		c.cacheBusted = true
  		return "", nil
  	}
  	logrus.Debugf("[BUILDER] Use cached version: %s", runConfig.Cmd)
  	return cacheID, nil
  }
docs: // Probe checks if cache match can be found for current build instruction.
  // It returns the cachedID if there is a hit, and the empty string on miss