refactor(docker): perform Webpack asset compilation eagerly

While it is simpler to perform asset compilation with Webpack after
the contents of the `cl` directory are copied, doing this comes at
a cost: the layer generated by Webpack is invalidated any time that
the contents of `cl` change. This happens much more frequently than
the compiled `react.js` and its dependencies change.

docker/django/Dockerfile

@@ -44,20 +44,23 @@ ARG BUILD_ENV=prod
 FROM build-base AS python-base
 
 WORKDIR $PYSETUP_PATH
-
 COPY poetry.lock pyproject.toml ./
 RUN poetry install --no-root $(test "$BUILD_ENV" != "dev" && echo "--without dev")
 
-COPY . /opt/courtlistener
-
-
 # Collect compiled assets from webpack-build stage.
 # Note: Always produce production code and never produce source-maps
 # (run the devMiddleware for legible output)
+# Copy a subset of the `cl` directory so that this compilation
+# can be cached in a layer that precedes the full directory copy.
 WORKDIR /opt/courtlistener/cl
+COPY ./cl/package.json ./cl/package-lock.json ./
+COPY ./cl/*.config.js ./cl/tsconfig.json ./
+COPY ./cl/assets ./assets
 RUN npm install
 RUN npx webpack --mode=production --no-devtool
 
+COPY . /opt/courtlistener
+
 WORKDIR /opt
 
 # We log to stdout by default, but we have a config for logging here. Even if