CentOS 6 和 RHEL 6 不再支持，它们的最后一个 build 是 docker 1.7.1.

CentOS 6 and RHEL 6 are no longer supported, and the last build for them is docker 1.7.1.

文档的那个页面(https://docs.docker.com/engine/installation/centos/) 还提到；

That page of the documentation (https://docs.docker.com/engine/installation/centos/) also mentions;

Docker 在 CentOS 7.X 上运行无论您的 CentOS 版本如何，Docker 都需要 64 位安装.此外，您的内核必须至少为 CentOS 7 运行的 3.10.

Docker runs on CentOS 7.X Docker requires a 64-bit installation regardless of your CentOS version. Also, your kernel must be 3.10 at minimum, which CentOS 7 runs.

运行这些发行版 (2.6.x) 的内核已经超过 13 年了，尽管 Red Hat 向后移植了新功能，但它们缺少 Docker 所需的某些选项，并且已证明不稳定，不适合生产.

The kernel that those distro's are running on (2.6.x) is over 13 years old, and although newer features are back-ported to them by Red Hat, they lack certain options that are required by Docker, and have proven to be unstable, and unsuitable for production.

如果你想(继续)使用 Docker，我鼓励你升级升级到 CentOS 7.x.

I encourage you to upgrade upgrade to CentOS 7.x if you want to (keep) using Docker.