Image Builds: Stop Busting the Layer Cache

The Dockerfile at /root/case/Dockerfile rebuilds slowly: it copies the whole app (COPY . .) BEFORE installing dependencies, so any source edit changes that early layer and busts the cache for the pip install below it — every build re-runs the install. Reorder the Dockerfile to the cache-friendly pattern: copy only the dependency manifest (requirements.txt) and run the install FIRST, then copy the rest of the source. Grading reads the Dockerfile, so fix the file in place.