Update dockerfile to using bookworm, and to avoid invalidating cache unless updating requirements.

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.12-slim-bullseye
+FROM python:3.12-slim-bookworm
 
 # PYTHONFAULTHANDLER: Propagate tracebacks from all threads.
 # PYTHONUNBUFFERED: Write terminal output straight to docker (to not confuse Docker Compose).
@@ -13,14 +13,17 @@ ENV PYTHONFAULTHANDLER=1 \
   PIP_DISABLE_PIP_VERSION_CHECK=on \
   PIP_DEFAULT_TIMEOUT=100
 ARG BUILD
-ENV BUILD ${BUILD}
+ENV BUILD=${BUILD}
 ARG REQUIREMENTS_FILE=requirements.txt
 
 WORKDIR /app
 
 RUN groupadd -g 1000 www && useradd -u 1000 -ms /bin/bash -g www www
-COPY --chown=www:www . .
-RUN mkdir /app/src/static && \
+
+# Only copy the requirements file first to leverage Docker cache
+COPY $REQUIREMENTS_FILE .
+
+RUN mkdir -p /app/src/static && \
     chown www:www /app/src/static && \
     apt-get update && \
     apt-get install -y \
@@ -35,8 +38,12 @@ RUN mkdir /app/src/static && \
      libffi-dev \
      shared-mime-info \
      gettext && \
-    pip install --no-cache-dir -r $REQUIREMENTS_FILE && \
-    django-admin compilemessages
+    pip install --no-cache-dir -r $REQUIREMENTS_FILE
+
+# Copy the rest of the application
+COPY . .
+
+RUN django-admin compilemessages
 
 ENTRYPOINT ["./entrypoint.sh"]