pkgs.dockerTools.mergeDrvs

The following is the current implementation of this function.

mergeDrvs =
    {
      derivations,
      onlyDeps ? false,
    }:
    runCommand "merge-drvs"
      {
        inherit derivations onlyDeps;
      }
      ''
        if [[ -n "$onlyDeps" ]]; then
          echo $derivations > $out
          exit 0
        fi

        mkdir $out
        for derivation in $derivations; do
          echo "Merging $derivation..."
          if [[ -d "$derivation" ]]; then
            # If it's a directory, copy all of its contents into $out.
            cp -drf --preserve=mode -f $derivation/* $out/
          else
            # Otherwise treat the derivation as a tarball and extract it
            # into $out.
            tar -C $out -xpf $drv || true
          fi
        done
      '';