Dockerfile: copy requirements.txt after apt

This way, modifications to requirements.txt will not trigger apt-get,
slightly reducing wait times during development when adding or updating
requirements.

Dockerfile

@@ -2,11 +2,11 @@ FROM python:3.12.0-slim-bookworm
 
 WORKDIR /app
 
-COPY requirements.txt .
-
 # Install gcc temporarily until wheels for httptools on Python 3.12 are available
 RUN apt-get update && apt-get install --no-install-recommends -y gcc libc6-dev libmagic1 && rm -rf /var/lib/apt/lists/*
 
+COPY requirements.txt .
+
 RUN --mount=type=cache,target=/root/.cache pip install -r requirements.txt
 
 COPY . .