Author doesn’t mention it but I wonder if tried or considered Nix/NixOS’s reproducible developer environments and ruled them out for any reason. I couldn’t tell from the article if there’s something unique to his requirements that disqualifies them.
Nix solves a different problem than Hocus. Nix lets you define a development environment, Hocus gives you a way to run it on a remote server. Right now we use Dockerfiles to let users define the packages they need in their dev env, but we would like to support Nix in the future too. Interestingly, you can use custom BuildKit syntax https://docs.docker.com/build/dockerfile/frontend/ to build Nix environments with Docker https://github.com/reproducible-containers/buildkit-nix, and that's probably what we will end up supporting.
I think Nix is relevant here, because being able to run software across different machines reproducibly is one of its major selling point. I particularly like that it doesn't rely on virtualization or containerization to do that. It's up to the user to decide how to isolate the runtime environment from the host or whether they even should. Alternatively, tools building upon Nix can make that decision for them. Either way, it allows for a more flexible approach when you have to weigh the pros and cons of different isolation strategies. Development environments defined by Nix tend to compose well too, as a result of this design.
https://nixos.org/explore
https://nix.dev/