Should this really be on this site? I'm half tempted to delete the post.
That said:
Regarding Dockerization early... I'm not entirely sure I agree... while down time is bad, lifting/shifting the database to a bigger server isn't too hard... same goes for the website.
You can get to be incredibly huge on a single database server, and a single application server if the code is well written to support that model... Stack Overflow grew vertically a *LOT* before they needed more than 2 servers.
Personally, I'd say Dockerize early for a more streamlined development CI/CD process. Depending on the database and application chosen, closer to bare metal may be a wiser choice. If you really want to invest in multiple DB servers up front determine what your needs are. Is it absolute scaling, or is it read-redundancy? These things should shape decisions.