Make pip install faster in Docker build for Complement testing (#9610)

Make pip install faster in Docker build for [Complement](https://github.com/matrix-org/complement) testing.

If files have changed in a `COPY` command, Docker will invalidate all of the layers below. So I changed the order of operations to install all dependencies before we `COPY synapse /synapse/synapse/`. This allows Docker to use our cached layer of dependencies even when we change the source of Synapse and speed up builds dramatically! `53.5s` -> `3.7s` builds 

As an alternative, I did try using BuildKit caches but this still took 30 seconds overall on that step. 15 seconds to gather the dependencies from the cache and another 15 seconds to `Installing collected packages`.

Fix https://github.com/matrix-org/synapse/issues/9364

changelog.d/9610.docker

+Speed up Docker builds and make it nicer to test against Complement while developing (install all dependencies before copying the project).

docker/Dockerfile

@@ -25,42 +25,40 @@ LABEL org.opencontainers.image.licenses='Apache-2.0'
 
 # install the OS build deps
 RUN apt-get update && apt-get install -y \
-        build-essential \
-        libffi-dev \
-        libjpeg-dev \
-        libpq-dev \
-        libssl-dev \
-        libwebp-dev \
-        libxml++2.6-dev \
-        libxslt1-dev \
-        openssl \
-        rustc \
-        zlib1g-dev \
-        && rm -rf /var/lib/apt/lists/*
-
-# Build dependencies that are not available as wheels, to speed up rebuilds
-RUN pip install --prefix="/install" --no-warn-script-location \
-        cryptography \
-        frozendict \
-        jaeger-client \
-        opentracing \
-        # Match the version constraints of Synapse
-        "prometheus_client>=0.4.0" \
-        psycopg2 \
-        pycparser \
-        pyrsistent \
-        pyyaml \
-        simplejson \
-        threadloop \
-        thrift
-
-# now install synapse and all of the python deps to /install.
-COPY synapse /synapse/synapse/
+    build-essential \
+    libffi-dev \
+    libjpeg-dev \
+    libpq-dev \
+    libssl-dev \
+    libwebp-dev \
+    libxml++2.6-dev \
+    libxslt1-dev \
+    openssl \
+    rustc \
+    zlib1g-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy just what we need to pip install
 COPY scripts /synapse/scripts/
 COPY MANIFEST.in README.rst setup.py synctl /synapse/
+COPY synapse/__init__.py /synapse/synapse/__init__.py
+COPY synapse/python_dependencies.py /synapse/synapse/python_dependencies.py
 
+# To speed up rebuilds, install all of the dependencies before we copy over
+# the whole synapse project so that we this layer in the Docker cache can be
+# used while you develop on the source
+#
+# This is aiming at installing the `install_requires` and `extras_require` from `setup.py`
 RUN pip install --prefix="/install" --no-warn-script-location \
-        /synapse[all]
+    /synapse[all]
+
+# Copy over the rest of the project
+COPY synapse /synapse/synapse/
+
+# Install the synapse package itself and all of its children packages.
+#
+# This is aiming at installing only the `packages=find_packages(...)` from `setup.py
+RUN pip install --prefix="/install" --no-deps --no-warn-script-location /synapse
 
 ###
 ### Stage 1: runtime
@@ -69,16 +67,16 @@ RUN pip install --prefix="/install" --no-warn-script-location \
 FROM docker.io/python:${PYTHON_VERSION}-slim
 
 RUN apt-get update && apt-get install -y \
-        curl \
-        gosu \
-        libjpeg62-turbo \
-        libpq5 \
-        libwebp6 \
-        xmlsec1 \
-        libjemalloc2 \
-        libssl-dev \
-        openssl \
-        && rm -rf /var/lib/apt/lists/*
+    curl \
+    gosu \
+    libjpeg62-turbo \
+    libpq5 \
+    libwebp6 \
+    xmlsec1 \
+    libjemalloc2 \
+    libssl-dev \
+    openssl \
+    && rm -rf /var/lib/apt/lists/*
 
 COPY --from=builder /install /usr/local
 COPY ./docker/start.py /start.py
@@ -91,4 +89,4 @@ EXPOSE 8008/tcp 8009/tcp 8448/tcp
 ENTRYPOINT ["/start.py"]
 
 HEALTHCHECK --interval=1m --timeout=5s \
-        CMD curl -fSs http://localhost:8008/health || exit 1
+    CMD curl -fSs http://localhost:8008/health || exit 1