The biggest problem with any CI system is that you need an execution environment. Changing this environment should be the same as changing the code. Docker (or rather podman) has given us the tools to do this.

Now if CI systems would allow me to build that container image myself, I could pretty much guarantee that local build/tests and CI build/tests can run inside the same environment. I hacked something like this for gitlab but it's ugly and slow.

So in conclusion, I think that CI systems should expect container creation, some execution inside that container, and finally some reporting or artifact generation from the results.