Can we install docker on CentOS6.8 to install Discourse?
If you can make docker work. I have seen many reports of problems with Centos, especially old versions.
It is much easier to install Discourse on its on server, and if you’re doing that there is no reason not to use an up to date os.
If you want it to be easy, use Ubuntu.
If you insist on an old version of Centos you should budget some hours to search the web for how to get docker working before you start to install Discourse.