Correct licensing metadata on the Docker image (#17141)

### 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/17141.docker

@@ -0,0 +1 @@
+Correct licensing metadata on Docker image.

docker/Dockerfile

@@ -163,7 +163,7 @@ FROM docker.io/library/python:${PYTHON_VERSION}-slim-bookworm
 LABEL org.opencontainers.image.url='https://matrix.org/docs/projects/server/synapse'
 LABEL org.opencontainers.image.documentation='https://github.com/element-hq/synapse/blob/master/docker/README.md'
 LABEL org.opencontainers.image.source='https://github.com/element-hq/synapse.git'
-LABEL org.opencontainers.image.licenses='Apache-2.0'
+LABEL org.opencontainers.image.licenses='AGPL-3.0-or-later'
 
 RUN \
   --mount=type=cache,target=/var/cache/apt,sharing=locked \