Include opencontainers labels in Docker image (#9612)

Cf. https://github.com/opencontainers/image-spec/blob/master/annotations.md#pre-defined-annotation-keys

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

changelog.d/9612.docker

@@ -0,0 +1 @@
+Include [opencontainers labels](https://github.com/opencontainers/image-spec/blob/master/annotations.md#pre-defined-annotation-keys) in the Docker image.

docker/Dockerfile

@@ -18,6 +18,11 @@ 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 \