complement-synapse: COPY existing dir from base (#18294)

The base postgres image already has the /var/run/postgresql directory,
and COPY can set file ownership with chown=, so COPY it instead of
making it from scratch & manually setting its ownership.

### 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/18294.docker

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

docker/complement/Dockerfile

@@ -25,7 +25,7 @@ FROM $FROM
 RUN adduser --system --uid 999 postgres --home /var/lib/postgresql
 COPY --from=postgres_base /usr/lib/postgresql /usr/lib/postgresql
 COPY --from=postgres_base /usr/share/postgresql /usr/share/postgresql
-RUN mkdir /var/run/postgresql && chown postgres /var/run/postgresql
+COPY --from=postgres_base --chown=postgres /var/run/postgresql /var/run/postgresql
 ENV PATH="${PATH}:/usr/lib/postgresql/13/bin"
 ENV PGDATA=/var/lib/postgresql/data