Dockerfile: add source annotation (#4626)

`org.opencontainers.image.source` URL to get source code for building the image (string)

https://github.com/opencontainers/image-spec/blob/main/annotations.md

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

Dockerfile

@@ -7,6 +7,7 @@ ARG BASE
 FROM ${BASE}
 
 LABEL org.opencontainers.image.authors="Torin Sandall <torinsandall@gmail.com>"
+LABEL org.opencontainers.image.source="https://github.com/open-policy-agent/opa"
 
 # Temporarily allow us to identify whether running from within an offical
 # Docker image, so that we may print a warning when uid or gid == 0 (root)