Docker build failure: could not resolve dependency `python3-dev` on Debian Bullseye (amd64) — getting E: Package 'python3-dev' has no installation candidate

I'm building a Python Docker image using FROM python:3.9-slim-bullseye and running apt-get update && apt-get install -y python3-dev build-essential. The build fails with "Package python3-dev has no installation candidate." I suspect Bullseye might have changed how it handles this package or the slim image is missing something. Any ideas?

i keep seeing this error and i dont understand what it means -- what does "no installation candidate" even mean? like is python3-dev supposed to be there or something

im trying to build a simple flask app with docker on bullseye and every time i run the build command it just dies at the pip install step. google says use python:3.9 as the base instead of debian but then i have to rewrite my whole dockerfile which was already annoying

does anyone know

wait so does this mean i can't use python at all?? my dockerfile literally just says pip install and it keeps throwing that error. is bullseye old? google kept telling me to update but i dont know how you update the whole os in a container. anyone helped with this -- im stuck on one line of code for 2 hours