Make builder script more variable robust by using parameters and catching corner cases

If $ADD_MIRROR wasn't set, we end up with an empty .list file.

If $SUITE wasn't set, we wouldn't fail explicitly.

Stop pushing images based on $SUITE otherwise we push a bullseye image for bullseye-backports

$RUNTIME was hard-coded, and could be separated out so different environments don't have to suffer TPOs weird runner situations.

If there was not an initial image in the registry, the script would fail.