make rootfs tarball for wsl or other purposes from a frozen docker image export

mkrootfstar.sh

#!/bin/bash

### expects gzip, jq, and tar
### converts an image downloaded with the moby image download script into a rootfs.tar.gz file.
### get the moby script from https://github.com/moby/moby/blob/master/contrib/download-frozen-image-v2.sh

set -x

ROOT="$1"

if [ x"$ROOT" = x ];then
        echo "please provide a root as arg 1"
        exit 1
fi

find_child_image_with_parent() {
    if [ x"$1" = x -o x"$1" = xnull ];then
            p=null
    else
            p='"'"$1"'"'
    fi
    jq -r --argjson p "$p" '.|select(.parent == $p)|.id' "$ROOT"/????????????????????????????????????????????????????????????????/json
}

TMPROOT="$ROOT/.$$-tmp"

if [ x"${TMPROOT%/*}" = x ];then
        echo "root can't be /"
        exit 1
fi

rm -rf "$TMPROOT"
mkdir -p "$TMPROOT"
trap "rm -rf \"$TMPROOT\"" EXIT

P=null
while true;do
    C=`find_child_image_with_parent "$P"`
    [ x"$C" = x ] && break
    echo "$C"
    gzip -dcf "$ROOT/$C/layer.tar" | (cd "$TMPROOT";tar x)
    P="$C"
done

(cd $TMPROOT;tar zcf ../rootfs.tar.gz .)