Challenge 2: Reorder for Better Caching — Possible Solution
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COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .

WHY THIS WORKS AS AN ANSWER
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requirements.txt (the dependency manifest) is copied FIRST, on its
own, followed immediately by pip install — this means the pip install
layer's cache only gets invalidated when requirements.txt itself
actually changes, exactly the same principle this chapter's npm
example demonstrated.

The full COPY . . (bringing in the rest of the application's source
code) now happens LAST, after dependencies are already installed. A
change to any Python source file only invalidates this final COPY
layer — the pip install layer above it stays cached and is reused,
since nothing about requirements.txt changed. Compare this to the
original ordering, where COPY . . came first: any source change would
invalidate that layer, and therefore force pip install to re-run on
literally every build regardless of whether dependencies were ever
touched.