Use uv to install packages in Docker

In order to do this we need to install uv inside the Docker container.
We don't want it being installed in the venv that we use for production.
So we need to do this as a separate step.

Part of the reason uv is faster is because it uses a cache of packages.
By default it uses `$HOME/.cache/uv` as the location for this cache
(even if you use --no-cache, it makes a temp folder in there)[^1].

Our docker image doesn't have permission to write there, so we override
the default and set the cache to be in /tmp/ instead

[^1] https://docs.astral.sh/uv/concepts/cache/#cache-directory

docker/Dockerfile

@@ -3,6 +3,8 @@ FROM python:3.11-slim-bookworm as base
 ENV CLAMAV_MIRROR_URL https://s3.eu-west-1.amazonaws.com/notifications.service.gov.uk-clamav-database-mirror/clam
 ENV PYTHONUNBUFFERED=1
 ENV DEBIAN_FRONTEND=noninteractive
+ENV UV_CACHE_DIR='/tmp/uv-cache/'
+
 
 # Use clamav database from private mirror. Disable with: --build-arg CLAMAV_USE_MIRROR=false for local builds
 ARG CLAMAV_USE_MIRROR=true
@@ -52,9 +54,11 @@ RUN echo "Install OS dependencies for python app requirements" && \
 
 COPY requirements.txt .
 
+RUN pip install uv
+
 RUN echo "Installing python requirements" && \
     python3 -m venv /opt/venv && \
-    /opt/venv/bin/pip install -r requirements.txt
+    uv pip sync --python /opt/venv/bin/python requirements.txt
 
 COPY . .
 RUN make generate-version-file  # This file gets copied across
@@ -100,6 +104,7 @@ USER notify
 RUN mkdir -p app
 
 # Install dev/test requirements
+RUN pip install uv
 COPY --chown=notify:notify requirements_for_test.txt ./
 RUN make bootstrap