Make pip install faster for Complement testing

Install dependencies before we copy over the whole project
so that any change we make to the project source while developing
does not invalidate the Docker layer cache where we installed
all of the dependencies. This speeds up Docker rebuilds by a lot!

changelog.d/9610.docker

@@ -0,0 +1 @@
+Speed up Docker builds and make it nicer to test against Complement while developing (install all dependencies before copying the project).

docker/Dockerfile

@@ -30,31 +30,29 @@ RUN apt-get update && apt-get install -y \
     libxslt1-dev \
     rustc \
     zlib1g-dev \
- && rm -rf /var/lib/apt/lists/*
+    && 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/
+# 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
@@ -70,7 +68,7 @@ RUN apt-get update && apt-get install -y \
     libwebp6 \
     xmlsec1 \
     libjemalloc2 \
- && rm -rf /var/lib/apt/lists/*
+    && rm -rf /var/lib/apt/lists/*
 
 COPY --from=builder /install /usr/local
 COPY ./docker/start.py /start.py
@@ -83,4 +81,4 @@ 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