Add a separate build stage for things that need compiling

Dockerfile

@@ -1,13 +1,25 @@
-FROM ghcr.io/praekeltfoundation/pypy-base-nw:2-buster
+FROM ghcr.io/praekeltfoundation/pypy-base-nw:2-buster AS builder
 
-RUN addgroup --system vumi \
-    && adduser --system --ingroup vumi vumi
+RUN apt-get update
+RUN apt-get -yy install build-essential libssl-dev libffi-dev
 
+COPY requirements.txt /requirements.txt
+
+RUN pip install --upgrade pip
 # We need the backport of the typing module to build Twisted.
 RUN pip install typing==3.10.0.0
 
-COPY requirements.txt /requirements.txt
-RUN pip install -r /requirements.txt
+RUN pip wheel -w /wheels -r /requirements.txt
+
+
+FROM ghcr.io/praekeltfoundation/pypy-base-nw:2-buster
+
+COPY ./requirements.txt /requirements.txt
+COPY --from=builder /wheels /wheels
+RUN pip install -f /wheels -r /requirements.txt
+
+RUN addgroup --system vumi \
+    && adduser --system --ingroup vumi vumi
 
 WORKDIR /app