> Docker is for running applications

Yes but no. Docker is mainly used for its ability to build and reuse standardised containers. It’s very much a distribution mechanism (I mean that’s why it’s called docker). From other comments I get the feeling that NixOS could be used to build reproducible containers in a way that remind me of building tools like Bazel but it’s not entirely clear to me why you would chose to do that nor if it’s actually a goal of the product.