[Nix] Docker image must carry the Admin WebUI

- first create the Nix UI derivation (./nix/ui.nix). Follow the existing https://github.com/Cosmian/kms/blob/develop/nix/ui.nix.
- add a dependency to docker.nix in order to build ui.nix
- test_docker_image.sh must be adapted to test the availability of the UI (/health endpoint)