The image of kubernetes pod is based from image python3.8-slim. This image is based from Debian 12. And upx binary (upx-ucl package provides it) only is available in: buster, bulleye and sid.
FROM python:3.8-slim AS build
ADD requirements.txt /
ADD baculatar.py /
-RUN apt-get update && apt-get -y install binutils upx && pip3 install -r requirements.txt
+RUN apt-get update && apt-get -y install binutils && pip3 install -r requirements.txt
RUN pyinstaller --onefile baculatar.py
FROM python:3.8-slim