Optimize Dockerfile-workers (#18292)

- Use a `uv:python` image for the first build layer, to reduce the
number of intermediate images required, as the
main Dockerfile uses that image already
- Use a cache mount for `apt` commands
- Skip a pointless install of `redis-server`, since the redis Docker
image is copied from instead
- Move some RUN steps out of the final image layer & into the build
layer

Depends on https://github.com/element-hq/synapse/pull/18275

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/18292.docker

@@ -0,0 +1 @@
+Optimize the build of the workers image.

docker/Dockerfile-workers

@@ -3,18 +3,37 @@
 ARG SYNAPSE_VERSION=latest
 ARG FROM=matrixdotorg/synapse:$SYNAPSE_VERSION
 ARG DEBIAN_VERSION=bookworm
+ARG PYTHON_VERSION=3.12
 
-# first of all, we create a base image with an nginx which we can copy into the
+# first of all, we create a base image with dependencies which we can copy into the
 # target image. For repeated rebuilds, this is much faster than apt installing
 # each time.
 
-FROM docker.io/library/debian:${DEBIAN_VERSION}-slim AS deps_base
+FROM ghcr.io/astral-sh/uv:python${PYTHON_VERSION}-${DEBIAN_VERSION} AS deps_base
+
+    # Tell apt to keep downloaded package files, as we're using cache mounts.
+    RUN rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
+
     RUN \
        --mount=type=cache,target=/var/cache/apt,sharing=locked \
        --mount=type=cache,target=/var/lib/apt,sharing=locked \
       apt-get update -qq && \
       DEBIAN_FRONTEND=noninteractive apt-get install -yqq --no-install-recommends \
-          redis-server nginx-light
+          nginx-light
+
+    RUN \
+    # remove default page
+      rm /etc/nginx/sites-enabled/default && \
+    # have nginx log to stderr/out
+      ln -sf /dev/stdout /var/log/nginx/access.log && \
+      ln -sf /dev/stderr /var/log/nginx/error.log
+
+    # --link-mode=copy silences a warning as uv isn't able to do hardlinks between its cache
+    # (mounted as --mount=type=cache) and the target directory.
+    RUN --mount=type=cache,target=/root/.cache/uv \
+      uv pip install --link-mode=copy --prefix="/uv/usr/local" supervisor~=4.2
+
+    RUN mkdir -p /uv/etc/supervisor/conf.d
 
 # Similarly, a base to copy the redis server from.
 #
@@ -27,31 +46,16 @@ FROM docker.io/library/redis:7-${DEBIAN_VERSION} AS redis_base
 # now build the final image, based on the the regular Synapse docker image
 FROM $FROM
 
-    # Install supervisord with uv pip instead of apt, to avoid installing a second
-    # copy of python.
-    # --link-mode=copy silences a warning as uv isn't able to do hardlinks between its cache
-    # (mounted as --mount=type=cache) and the target directory.
-    RUN \
-         --mount=type=bind,from=ghcr.io/astral-sh/uv:0.6.8,source=/uv,target=/uv \
-         --mount=type=cache,target=/root/.cache/uv \
-        /uv pip install --link-mode=copy --prefix="/usr/local" supervisor~=4.2
-
-    RUN mkdir -p /etc/supervisor/conf.d
-
-    # Copy over redis and nginx
+    # Copy over dependencies
     COPY --from=redis_base /usr/local/bin/redis-server /usr/local/bin
-
+    COPY --from=deps_base /uv /
     COPY --from=deps_base /usr/sbin/nginx /usr/sbin
     COPY --from=deps_base /usr/share/nginx /usr/share/nginx
     COPY --from=deps_base /usr/lib/nginx /usr/lib/nginx
     COPY --from=deps_base /etc/nginx /etc/nginx
-    RUN rm /etc/nginx/sites-enabled/default
-    RUN mkdir /var/log/nginx /var/lib/nginx
-    RUN chown www-data /var/lib/nginx
-
-    # have nginx log to stderr/out
-    RUN ln -sf /dev/stdout /var/log/nginx/access.log
-    RUN ln -sf /dev/stderr /var/log/nginx/error.log
+    COPY --from=deps_base /var/log/nginx /var/log/nginx
+    # chown to allow non-root user to write to http-*-temp-path dirs
+    COPY --from=deps_base --chown=www-data:root /var/lib/nginx /var/lib/nginx
 
     # Copy Synapse worker, nginx and supervisord configuration template files
     COPY ./docker/conf-workers/* /conf/