Backend Docker image fails to build: Debian 11 (bullseye) packages return 404

Description:
While trying to build the Docker image, it fails at apt-get install step.
The image python:3.11-slim-bullseye uses Debian 11, LTS support ended on Aug 31 (https://www.debian.org/News/2026/20260831), and several packages from bullseye-security now return 404.

How to reproduce:

1. Run docker compose up --build
2. The core build fails at apt-get install

The same 404s show up in CI, e.g. https://github.com/chaoss/CollectOSS/actions/runs/34911714170

Expected behavior:
The backend image builds.

Log files:
docker compose up --build on main (274301dc8), macOS arm64. apt-get update fetches the bullseye-security index, but downloads from the security pool return 404 (33 of the 111 packages, all from debian-security):

0.298 Get:2 http://deb.debian.org/debian-security bullseye-security InRelease [27.1 kB]
...
16.36 E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/p/perl/perl-base_5.32.1-4%2bdeb11u5_arm64.deb  404  Not Found [IP: 151.101.218.132 80]
16.36 E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/g/glibc/libc6_2.31-13%2bdeb11u14_arm64.deb  404  Not Found [IP: 151.101.218.132 80]
16.36 E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/g/git/git_2.30.2-1%2bdeb11u5_arm64.deb  404  Not Found [IP: 151.101.218.132 80]
16.36 E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/p/postgresql-13/libpq-dev_13.23-0%2bdeb11u4_arm64.deb  404  Not Found [IP: 151.101.218.132 80]
...
target core: failed to solve: process "/bin/sh -c set -x     && apt-get update     && apt-get -y install --no-install-recommends         git         bash         curl         gcc         software-properties-common         postgresql-contrib         musl-dev         wget         postgresql-client         libpq-dev         build-essential         tar         jq     && apt-get clean     && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*" did not complete successfully: exit code: 100

Software versions:

• CollectOSS: 274301dc8
• OS: macOS (Apple Silicon)

Possible fix:
Moving the base images to bookworm. Locally that also needed ENV UV_PYTHON=/usr/local/bin/python3.11, otherwise uv picks Debian's system Python (no headers) and psutil==5.8.0 fails to build. I would like to work on this one if the approach makes sense.

0.101 Using CPython 3.11.2 interpreter at: /usr/bin/python3.11
0.595   × Failed to build `psutil==5.8.0`
0.595       psutil/_psutil_common.c:9:10: fatal error: Python.h: No such file or
0.595       directory
0.595 hint: This error likely indicates that you need to install a library that provides "Python.h" for `psutil@5.8.0`

Image with bookworm + UV_PYTHON:

  Image collectoss:latest         Built                1.9s
core-1      | [7] collectoss.preflight [INFO] Python version: 3.11.16
core-1      | 2026-09-15 02:04:03 b6bfd66574a4 collectoss[34] INFO Gunicorn webserver started...

(The backend then stops on the empty GitHub API key I left in .env on purpose for #378 )

I used Claude to help analyze the build logs and draft this report. The builds and the fix were run and verified locally.