pkgs.dockerTools.mergeImages

The following is the current implementation of this function.

mergeImages = images: runCommand "merge-docker-images"
    {
      inherit images;
      nativeBuildInputs = [ file jq ]
        ++ compressors.none.nativeInputs
        ++ compressors.gz.nativeInputs
        ++ compressors.zstd.nativeInputs;
    } ''
    mkdir image inputs
    # Extract images
    repos=()
    manifests=()
    last_image_mime="application/gzip"
    for item in $images; do
      name=$(basename $item)
      mkdir inputs/$name

      last_image_mime=$(file --mime-type -b $item)
      case $last_image_mime in
        "application/x-tar") ${compressors.none.decompress};;
        "application/zstd") ${compressors.zstd.decompress};;
        "application/gzip") ${compressors.gz.decompress};;
        *) echo "error: unexpected layer type $last_image_mime" >&2; exit 1;;
      esac < $item | tar -xC inputs/$name

      if [ -f inputs/$name/repositories ]; then
        repos+=(inputs/$name/repositories)
      fi
      if [ -f inputs/$name/manifest.json ]; then
        manifests+=(inputs/$name/manifest.json)
      fi
    done
    # Copy all layers from input images to output image directory
    cp -R --update=none inputs/*/* image/
    # Merge repositories objects and manifests
    jq -s add "''${repos[@]}" > repositories
    jq -s add "''${manifests[@]}" > manifest.json
    # Replace output image repositories and manifest with merged versions
    mv repositories image/repositories
    mv manifest.json image/manifest.json
    # Create tarball and gzip
    tar -C image --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" --owner=0 --group=0 --xform s:'^./':: -c . | (
      case $last_image_mime in
        "application/x-tar") ${compressors.none.compress};;
        "application/zstd") ${compressors.zstd.compress};;
        "application/gzip") ${compressors.gz.compress};;
        # `*)` not needed; already checked.
      esac
    ) > $out
  '';