First stage compiles the code. This is good for isolation and reproducibility.
Second stage is a lightweight container to run the compiled binary.
Why is the author being attacked (by multiple comments) for not making things simpler when that was not claimed that as the goal. They are modernizing it.
Containers are good practice for CI/CD anyway.
Don't do what you don't need to do.
They are already long past the point of "complicate things unnecessarily".
A simple Dockerfile pales in comparison.
Docker is a (the, in some areas) modern way to do it, but far from the only way.
no, that's sandboxing.