Use Python 3.9 in Synapse dockerfiles by default (#12112)

changelog.d/12112.docker

+Use Python 3.9 in Docker images by default.
\ No newline at end of file

docker/Dockerfile

@@ -11,10 +11,10 @@
 # There is an optional PYTHON_VERSION build argument which sets the
 # version of python to build against: for example:
 #
-#    DOCKER_BUILDKIT=1 docker build -f docker/Dockerfile --build-arg PYTHON_VERSION=3.9 .
+#    DOCKER_BUILDKIT=1 docker build -f docker/Dockerfile --build-arg PYTHON_VERSION=3.10 .
 #
 
-ARG PYTHON_VERSION=3.8
+ARG PYTHON_VERSION=3.9
 
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 ### Stage 0: builder