Added storage of digest hash to reduce pulling if the image has not been updated

[JordanPlayz158]

diff docker-image-extract.original docker-image-extract 
231a232,237
> previousDigest=$(cat "$OUT_DIR/digest")
> if [ "$digest" = "$previousDigest" ]; then
>   echo "Docker digest is equal to previously extracted digest, exiting..."
>   exit 2
> fi
> 
286a293,295
> 
> echo "$digest" > "$OUT_DIR/digest"
> echo "Digest stored in $OUT_DIR/digest"

This is nice for automation of updating images/applications that only distribute binaries through a docker image, it not only makes docker hub get pulled from less if you do have a system like I do but it also exits with a certain code so you know it didn't error but it didn't get any new files so you don't need to restart the applications in question.

I would've made a repo but it was such a small change so I didn't think it was necessary. If needed, I can make a repo with the change and make a pull request instead.