code: func (agent *ecsAgent) verifyRequiredDockerVersion() (int, bool) {
  	supportedVersions := agent.dockerClient.SupportedVersions()
  	if len(supportedVersions) == 0 {
  		seelog.Critical("Could not get supported docker versions.")
  		return exitcodes.ExitError, false
  	}
  
  	// if api version 1.21 is supported, it means docker version is at least 1.9.0
  	for _, version := range supportedVersions {
  		if version == dockerclient.Version_1_21 {
  			return -1, true
  		}
  	}
  
  	// api 1.21 is not supported, docker version is older than 1.9.0
  	seelog.Criticalf("Required minimum docker API verion %s is not supported",
  		dockerclient.Version_1_21)
  	return exitcodes.ExitTerminal, false
  }
docs: // validateRequiredVersion validates docker version.
  // Minimum docker version supported is 1.9.0, maps to api version 1.21
  // see https://docs.docker.com/develop/sdk/#api-version-matrix