DockerはどうやってコンテナのIDを決定しているか

Dockerがコンテナを作成したときのIDをどうやって決定しているかを説明します。
結論から言うと、ランダムなIDを生成しています。

newContainer関数の中でgenerateIDAndNameが呼ばれ、同ファイルのgenerateID関数が呼ばれてここでランダムなIDが生成されています。

newContainer

daemon/container.go#L128

func (daemon *Daemon) newContainer(name string, operatingSystem string, config *containertypes.Config, hostConfig *containertypes.HostConfig, imgID image.ID, managed bool) (*container.Container, error) {
	var (
		id             string
		err            error
		noExplicitName = name == ""
	)
	id, name, err = daemon.generateIDAndName(name)
	if err != nil {
		return nil, err
	}

	if hostConfig.NetworkMode.IsHost() {
		if config.Hostname == "" {
			config.Hostname, err = os.Hostname()
			if err != nil {
				return nil, errdefs.System(err)
			}
		}
	} else {
		daemon.generateHostname(id, config)
	}
	entrypoint, args := daemon.getEntrypointAndArgs(config.Entrypoint, config.Cmd)

	base := daemon.newBaseContainer(id)
	base.Created = time.Now().UTC()
	base.Managed = managed
	base.Path = entrypoint
	base.Args = args //FIXME: de-duplicate from config
	base.Config = config
	base.HostConfig = &containertypes.HostConfig{}
	base.ImageID = imgID
	base.NetworkSettings = &network.Settings{IsAnonymousEndpoint: noExplicitName}
	base.Name = name
	base.Driver = daemon.imageService.GraphDriverForOS(operatingSystem)
	base.OS = operatingSystem
	return base, err
}

GenerateNonCryptoID

github.com/docker/docker/pkg/stringid/stringid.go#L69

// GenerateNonCryptoID generates unique id without using cryptographically
// secure sources of random.
// It helps you to save entropy.
func GenerateNonCryptoID() string {
	return generateID(readerFunc(rand.Read))
}

generateID

github.com/docker/docker/pkg/stringid/stringid.go#L44

func generateID(r io.Reader) string {
	b := make([]byte, 32)
	for {
		if _, err := io.ReadFull(r, b); err != nil {
			panic(err) // This shouldn't happen
		}
		id := hex.EncodeToString(b)
		// if we try to parse the truncated for as an int and we don't have
		// an error then the value is all numeric and causes issues when
		// used as a hostname. ref #3869
		if _, err := strconv.ParseInt(TruncateID(id), 10, 64); err == nil {
			continue
		}
		return id
	}
}