nix function to fetch docker image content

fetchDocker.nix

{ pkgs }:

{
  # fetch docker image content and return path in nix store
  # inspired from https://github.com/NixOS/nixpkgs/blob/6dab1b50a63fa6e9dfb9bd9eddd3f5d5c7b055ef/pkgs/build-support/docker/default.nix#L264
  fetchDocker = coordinates:
    let
      name = "${coordinates.finalImageName}-${coordinates.finalImageTag}";
      result = pkgs.dockerTools.runWithOverlay {
        name = "docker-exporter-${name}";
        diskSize = 512;
        fromImage = pkgs.dockerTools.pullImage coordinates;
        postMount = ''
          touch $out/json $out/VERSION
          tar -C mnt --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" -cf $out/layer.tar .
        '';
      };
    in pkgs.runCommandLocal "docker-export-${name}" { } ''
      mkdir $out
      tar -C $out -xf ${result}/layer.tar
    '';
}

/scratch/bocker2 via ❄️  impure (shell) took 17s 
❯ cat pkg_debian_docker.nix 
{ pkgs ? import <nixpkgs> {} }:

# Fetch from dockerhub via skopeo
pkgs.dockerTools.pullImage {
  # https://hub.docker.com/layers/library/debian/stable-20240812-slim/images/sha256-90128f59a7c6f6fdcb6493f587ea352d5c7507f52a6ddfba66fc56cd3d99dc2b
  imageName = "debian";
  finalImageTag = "stable-20240812-slim";
  imageDigest = "sha256:382967fd7c35a0899ca3146b0b73d0791478fba2f71020c7aa8c27e3a4f26672";

  # Extracted
  sha256 = "1grfplg28bw9kpgz733chf15l5k0h6f24crnhrcpqbxm6hzicgny";
}



/scratch/bocker2 via ❄️  impure (shell) 
❯ nix-build pkg_debian_docker.nix 
/nix/store/fxf7xprcq4rmq81mf07i5nq24wrmpnzb-docker-image-debian-stable-20240812-slim.tar