Do yourself a favor and avoid SMF if possible. It "works", but has terrible code quality (prime example [1]) and the addon/module system operates by diffing the original PHP source file and then patching in the needed changes. As you can imagine, this quickly leads to a lot of issues when updating or using multiple addons.
Ahh, it seems like they didn't have access to a router and just did everything within a super long procedure as a consequence, i've seen that very same anti-pattern many times and it's always a nightmare to work with and debug. I can easily imagine myself doing something like that 5 years ago: starting small but the project eventually growing in scope and this being the end result.
That said, at least the code is documented in some capacity with comments, which isn't always the case in some projects out there.
As for updating, as more time passes, i become more and more convinced that if you can't find software within a container where bumping versions is as easy as changing the image tag, then you'll run into problems of some sort sooner or later.
[1] https://github.com/SimpleMachines/SMF2.1/blob/release-2.1/So...