In Docker’s case is a non-issue because they were careful to use completely different names for all their packages. It’s only when the external repo uses the same names as the core that the dependency resolver can get confused.

Rant:

apt should either completely forbid external repos from using core package names (like Arch does), or look at both the package name and repo URL when deciding if a package is the same, not just package name.

I’m guessing that letting external repos “hijack” a package name was once upon a time seen as a feature and then they never got around to fixing it.