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
    '';