I don't understand what does it fix, also in #93. Because I don't understand the problem.
Can you explain

1. how you are building the image?
2. What output do you expect?
3. What output did you get?
4. A short explanation, if needed, why ${BASE_RELEASE_IMG_NAME%.img}.img" is better than IMG_FILENAME="${BASE_IMG_NAME}"?

Hi and a Merry Christmas for you and your Family.
Because english isnt my mothertongue, I'll try to explain what I meant as I could.

So, after I asked you by email where are the Descriptions are for Customizing the config file, I fiddled around with that.
After I understand how tho whole concept is to understand, I started to build my own Image.
I use the Docker Method with your custompios:devel Docker Image and latest RaspberryOS Image, because I am using Arch Linux and it is a Pita every update to fiddle around with docker-compose due Python Updates :( But thats not the Point.

I expected that, when I add BASE_RELEASE_IMG_NAME=mypersonal BASE_RELEASE_ZIP_NAME=mypersonalzip in config the Image would be renamed after building it.
but it doesnt.
My best way is to show code that I mean.

FILENAME=$(basename ls . | grep -e .img$ -e .raw$ | tail -n 1`)

if [ "${BASE_RELEASE_IMG_NAME}" == "default" ]; then
if [ "${BASE_DISTRO}" == "ubuntu" ]; then
CHANGE_STRING_LIST=( "ubuntu" )
else
CHANGE_STRING_LIST=( "raspbian" "raspios")
fi

# Handle variant name
for CHANGE_STRING in "${CHANGE_STRING_LIST[@]}"
do
    if [ -z "$1" ] || [ "$BASE_IGNORE_VARIANT_NAME" == "yes" ] ;then
        IMG_FILENAME=$(echo "${FILENAME::-4}"-"${DIST_VERSION}" | sed "s/${CHANGE_STRING}/${DIST_NAME,,}/").img
    else
        IMG_FILENAME=$(echo "${FILENAME::-4}"-"${DIST_VERSION}" | sed "s/${CHANGE_STRING}/${DIST_NAME,,}-$1/").img
    fi
done

else
IMG_FILENAME="${BASE_IMG_NAME}"
fi

mv "${FILENAME}" "${IMG_FILENAME}"

if [ "${BASE_RELEASE_COMPRESS}" == "yes" ]; then
echo "Compressing image"
if [ "${BASE_RELEASE_IMG_NAME}" == "default" ]; then
zip "${IMG_FILENAME::-4}.zip" "${IMG_FILENAME}"
else
zip "${BASE_IMG_NAME%.zip}".zip "${IMG_FILENAME}"
fi
else
echo "Not compressing because setting is disabled"
fi`

in the portion if [ "${BASE_RELEASE_IMG_NAME}" == "default" ]; then

you test for other Variable Content then "default", if it is on default the outcome is how it is expected.
the Image named in my case 2020-12-02-mypersonalos-buster-lite.img

But if I set those variables to my wanted names the image name is the same as my Input image.
So I greped the release script an thought about it.
Then I figured out that it is not "default" you set IMG_FILENAME="${BASE_IMG_NAME}"
but putting these variable in the config file has no effect. Same File Name as "default".
After that i wrote a little test script and as I expected I couldnt find where "${BASE_IMG_NAME}"
it gets parsed.
If you rename this to BASE_RELEASE_IMG_NAME you get the expected behavior to rename the IMG File to your liking.

This brought the Problem that thi zip file has the Name of the bare input file.
I hope that makes more sense to you.
Otherwise test the behavior by your self. and see what happends.
Another thing that brought me to this is - name: Copy output image run: cp ${{ github.workspace }}/repository/src/workspace/*-raspios-*-lite.img mainsailos-raspios-lite-latest.img

from a github - workflow .yml file.
This has BASE* Vars set but they had the Line above as Workaround through false renaming.

Regards Stephan

Ah and I forget: ${BASE_RELEASE_IMG_NAME%.img}.img" is the savest way to prevent a double .img at the end if someone ( is stupid like me) an puts filename extension in the VAR mistakenly ;)

Got it, thanks for the contribution!