Dockerfile: Split build into multiple stages

Currently the Dockerfile has a single RUN directive with all the needed
commands in it. This optimizes for image size by not creating too many
"layers" (which are only additive). However it means the result that gets
cached can basically never be reused, because any change to the source code
will need to execute the whole RUN block again.

This commit switches to a docker "multi-stage" build, where we have a build
image that is separate from the final one that gets published. The build
image can be cached locally, and size is no longer a significant concern.

This approach allows the single RUN command to be split up into multiple RUN
commands that only execute when strictly needed (i.e. when their result
is not cached locally).

This drastically improves the build time when rebuilding the image after
a simple code change, because the build image doesn't have to install all
the apt packages, for example. This leads to a nicer developer experience
when using docker locally for development and testing.

Dockerfile

@@ -1,7 +1,13 @@
-FROM debian:buster-slim
+FROM debian:buster-slim AS build
 
-ARG BUILD_SERIES=dev
+RUN set -eu; \
-ARG BUILD_ID=0
+    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 +17,39 @@ COPY babel.cfg /opt/snikket-web-portal/babel.cfg
 
 WORKDIR /opt/snikket-web-portal
 
-RUN set -eu; \
+RUN pip3 install -r requirements.txt; \
-    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; \
     pip3 install -r build-requirements.txt; \
-    make; \
+    make;
-    pip3 uninstall -yr build-requirements.txt; \
+
-    apt-get remove -y build-essential make libpython3-dev; \
+
-    apt-get autoremove -y; \
+FROM debian:buster-slim
-    pip3 install hypercorn; \
+
-    rm -rf /root/.cache; \
+ARG BUILD_SERIES=dev
-    apt-get clean ; rm -rf /var/lib/apt/lists
+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; \
+    pip3 install hypercorn;
+
+WORKDIR /opt/snikket-web-portal
+
+COPY requirements.txt /opt/snikket-web-portal/requirements.txt
+RUN pip3 install -r requirements.txt;
+
+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