Draft: Minimal Changes Necessary to Use a Nix Flake

What does this Merge Request do?

Currently only starts rails-web, rails-background-jobs and webpack via nix. More services are dependent on managing their dependencies via nix. Easy next targets are redis and postgres, pretty much everything that is a binary that is vendored and does not need to be built.

Also probably MacOS only, would need to do some testing on Linux to ensure the right dependencies are passed in.

Testing this requires:

1. Installing nix
2. Enabling nix flakes
3. Checking out gitlab!54008 (closed)
4. Checking out gitlab-pages!467 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.