Put opencontainers labels to the final image (#9765)

They don't make any sense on the intermediate builder image. The final
images needs them to be of use for anyone.

Signed-off-by: Johannes Wienke <languitar@semipol.de>

changelog.d/9765.docker

@@ -0,0 +1 @@
+Move opencontainers labels to the final Docker image such that users can inspect them.

docker/Dockerfile

@@ -18,11 +18,6 @@ ARG PYTHON_VERSION=3.8
 ###
 FROM docker.io/python:${PYTHON_VERSION}-slim as builder
 
-LABEL org.opencontainers.image.url='https://matrix.org/docs/projects/server/synapse'
-LABEL org.opencontainers.image.documentation='/~https://github.com/matrix-org/synapse/blob/master/docker/README.md'
-LABEL org.opencontainers.image.source='/~https://github.com/matrix-org/synapse.git'
-LABEL org.opencontainers.image.licenses='Apache-2.0'
-
 # install the OS build deps
 RUN apt-get update && apt-get install -y \
     build-essential \
@@ -66,6 +61,11 @@ RUN pip install --prefix="/install" --no-deps --no-warn-script-location /synapse
 
 FROM docker.io/python:${PYTHON_VERSION}-slim
 
+LABEL org.opencontainers.image.url='https://matrix.org/docs/projects/server/synapse'
+LABEL org.opencontainers.image.documentation='/~https://github.com/matrix-org/synapse/blob/master/docker/README.md'
+LABEL org.opencontainers.image.source='/~https://github.com/matrix-org/synapse.git'
+LABEL org.opencontainers.image.licenses='Apache-2.0'
+
 RUN apt-get update && apt-get install -y \
     curl \
     gosu \