It’s true we don’t currently have good user documentation for Plone 6 but we are working on it.
Interesting question, how to change the admin password! You do that by going to the back end, e.g. localhost:8080/manage then acl_users (i don’t have it in front of me to give you a more specific answer) because admin is a Zope user, not a Plone user.
To put a blog on the internet, we have several tutorials though they’re in need of updating. I posted one here earlier … I will find it
When you say you’d like to know how to get a blog on the internet, are you asking how to create a server and install Plone on it? Or more about how to configure Plone so it can serve as a blog?
Since you say you want to customize it, I'd recommend " * Method 2: Install using packages - for developing custom applications, themes, and add-ons for Plone."
It depends on what kind of customization you mean. Simple things are settings you can change. Others require installation of a Volto theme or addon. Others require development. Doing that with a Docker setup is more complicated. Maybe others can answer this question better.