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

@@ -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

@@ -38,30 +38,28 @@ RUN apt-get update && apt-get install -y \
     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/
+# 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]
 
+# 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
+
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