refactor: Use debian-slim image instead of python image

Some companies only have access to the general debian images,
not to Python images.

As we're installing kubectl and python, the debian image
also provides more flexibility.

backend/Dockerfile

@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: Copyright DB Netz AG and the capella-collab-manager contributors
 # SPDX-License-Identifier: Apache-2.0
 
-ARG BASE_IMAGE=python:3.11-bookworm
+ARG BASE_IMAGE=debian:bookworm-slim
 FROM $BASE_IMAGE
 
 SHELL ["/bin/bash", "-euo", "pipefail", "-c"]
@@ -16,6 +16,10 @@ RUN apt-get update && \
     unzip \
     libpq-dev \
     curl \
+    python3 \
+    python3-pip \
+    python3-venv \
+    gnupg \
     && rm -rf /var/lib/apt/lists/*
 
 ARG KUBECTL_APT_REMOTE=https://apt.kubernetes.io/
@@ -32,6 +36,14 @@ COPY . /tmp/backend
 COPY .git_archival.txt /tmp/.git_archival.txt
 COPY startup.sh /opt/.startup.sh
 
+# Activate venv
+RUN ln -s $(which python3.11) /usr/bin/python && \
+    ln -sf $(which python3.11) /usr/bin/python3 && \
+    python -m venv /opt/.venv
+ENV _OLD_VIRTUAL_PATH="$PATH"
+ENV VIRTUAL_ENV=/opt/.venv
+ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+
 WORKDIR /tmp/backend
 RUN --mount=type=cache,id=capella-collaboration-backend-dependencies,target=/root/.cache/pip,sharing=locked \
     pip install ".[psycopg2]"

ci-templates/gitlab/image-builder.yml

@@ -26,7 +26,7 @@ variables:
   GUACAMOLE_IMAGE_NAME: "capella/collab/guacamole"
   DOCKER_BUILD_ARGS: "--no-cache"
   DOCKER_BUILDKIT: "1"
-  BASE_IMAGE: "debian:bullseye"
+  BASE_IMAGE: "debian:bookworm"
   KUBECTL_APT_REMOTE: "https://apt.kubernetes.io/"
 
 default: