chore(build): Add OCI label for container image source (#2541)

As specified in the OpenContainers Annotations Spec:
https://specs.opencontainers.org/image-spec/annotations/

Author: sklirg

docker/Dockerfile

@@ -13,6 +13,7 @@ RUN bash docker/build_operator.sh
 
 FROM ${BASE_IMAGE}
 LABEL maintainer="Team ACID @ Zalando <[email protected]>"
+LABEL org.opencontainers.image.source="https://github.com/zalando/postgres-operator"
 
 # We need root certificates to deal with teams api over https
 RUN apk --no-cache add curl