It's just given as an example of launching TKL docker container. Apologies if that wasn't clear. It has "E.g." just above so I thought it would be obvious but I should perhaps make it clearer that it's an example. [update] I edited the blog post so hopefully it's clearer that it's only an example. Feedback welcome...

When you first launch the container it should provide you with inithooks screens which you complete. Part of that process sets the root account password (and other passwords you may need).