I've been thinking a lot about this kind of thing recently - and put a prototype up of htvend [1] that allows you to archive out dependencies during an image build. The idea being that if you have a mix of private/public dependencies that the upstream dependencies can be saved off locally as blobs allowing your build process to be able to be re-run in the future, even if the upstream assets become unavailable (as appears to be the case here).
Their Dockerfiles include things like download pre built binaries from $SECRET_BASEURL which is hosted by them, can still be found in git log though. I imagine it will go offline / have auth soon enough.
Or if you have a decent sized deployment in one of the clouds, it's extremely likely you'll already use their internal registry (eg AWS ECR). I know that we do. So it's just a case of setting up a few docker build projects in git that push to your own internal registry.
