Reinstate cache deletion in multi-stage build

Previously, the multi-stage build increased the image size by about 30
MiB (163MiB -> 191MiB). Dropping the caches reduces the image size down
to 159MiB, leading to a net improvement of 4 MiB.

Dockerfile

@@ -1,7 +1,13 @@
-FROM debian:buster-slim
+FROM debian:buster-slim AS build
 
-ARG BUILD_SERIES=dev
-ARG BUILD_ID=0
+RUN set -eu; \
+    export DEBIAN_FRONTEND=noninteractive ; \
+    apt-get update ; \
+    apt-get install -y --no-install-recommends \
+        python3 python3-pip python3-setuptools python3-wheel \
+        libpython3-dev \
+        make build-essential \
+        netcat;
 
 COPY requirements.txt /opt/snikket-web-portal/requirements.txt
 COPY build-requirements.txt /opt/snikket-web-portal/build-requirements.txt
@@ -11,32 +17,41 @@ COPY babel.cfg /opt/snikket-web-portal/babel.cfg
 
 WORKDIR /opt/snikket-web-portal
 
-RUN set -eu; \
-    export DEBIAN_FRONTEND=noninteractive ; \
-    apt-get update ; \
-    apt-get install -y --no-install-recommends \
-        python3 python3-pip python3-setuptools python3-wheel \
-        libpython3-dev \
-        make build-essential \
-        netcat \
-        ; \
-    pip3 install -r requirements.txt; \
+RUN pip3 install -r requirements.txt; \
     pip3 install -r build-requirements.txt; \
-    make; \
-    pip3 uninstall -yr build-requirements.txt; \
-    apt-get remove -y build-essential make libpython3-dev; \
-    apt-get autoremove -y; \
-    pip3 install hypercorn; \
-    rm -rf /root/.cache; \
-    apt-get clean ; rm -rf /var/lib/apt/lists
+    make;
+
+
+FROM debian:buster-slim
+
+ARG BUILD_SERIES=dev
+ARG BUILD_ID=0
 
 COPY docker/env.py /etc/snikket-web-portal/env.py
+
 ENV SNIKKET_WEB_PYENV=/etc/snikket-web-portal/env.py
 
 ENV SNIKKET_WEB_PROSODY_ENDPOINT=http://127.0.0.1:5280/
 
 HEALTHCHECK CMD nc -zv ${SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_INTERFACE:-127.0.0.1} ${SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_PORT:-5765}
 
+RUN set -eu; \
+    export DEBIAN_FRONTEND=noninteractive ; \
+    apt-get update ; \
+    apt-get install -y --no-install-recommends \
+        python3 python3-pip python3-setuptools python3-wheel; \
+    apt-get clean ; rm -rf /var/lib/apt/lists; \
+    pip3 install hypercorn; \
+    rm -rf /root/.cache;
+
+WORKDIR /opt/snikket-web-portal
+
+COPY requirements.txt /opt/snikket-web-portal/requirements.txt
+RUN pip3 install -r requirements.txt; rm -rf /root/.cache;
+
+COPY --from=build /opt/snikket-web-portal/snikket_web/ /opt/snikket-web-portal/snikket_web
+COPY babel.cfg /opt/snikket-web-portal/babel.cfg
+
 RUN echo "$BUILD_SERIES $BUILD_ID" > /opt/snikket-web-portal/.app_version
 
 ADD docker/entrypoint.sh /entrypoint.sh