This warning has nothing to do with the bash script you use to build & install packages.
This warning orignates from command “dpkg-buildpackage” when it works on the source code to build the package.
Those who are conversant with mechanism of building packages, or the source code authors can explain this. Anyway as it is only a warning and not an error, and does not affect package building, I simply ignore it. I follow this saying “Dont worry, be happy” .