Stick a cache on your build server. As long as your cache doesn't go down you can cache all the build dependencies. Then github is not a dependency anymore.

That does not work with Docker in Docker Builds with the copy from pattern. (Especially not when running on sbt...)

Yes it does. You can pass http_proxy as a build argument to docker and then it will use your cache when it builds docker images.

Ditto decreased build speed because of internet downloads.

How far can you take this thinking?

"My app will build even with Github down!"

"My app will build with Github down, and the entire internet down!"

"My app will build with Github down, and the internet down, and the US government being taken over by a foreign military power!"

"My app will build with Github down, the internet down, and the US government being taken over, and Europe a smoldering crater from the last alien invasion!"

At some point - just - nobody cares that your app can build. There's bigger problems.

To me, "Github is down and not coming up!" is one of those "oh my god we now have serious problems who cares about our app" moments.

GitHub usually does a good job but we aren't paying for an SLA guarantee and major outages aren't unheard of. It's easy enough to make your build a deterministic function of the branch contents that I don't see much of a defense for not doing it.