Docker: added OCI image-spec labels.

Author: thresheek

pkg/docker/template.Dockerfile

@@ -1,6 +1,12 @@
 FROM @@CONTAINER@@
 
-LABEL maintainer="NGINX Docker Maintainers <[email protected]>"
+LABEL org.opencontainers.image.title="Unit"
+LABEL org.opencontainers.image.description="Official build of Unit for Docker."
+LABEL org.opencontainers.image.url="https://unit.nginx.org"
+LABEL org.opencontainers.image.source="https://github.com/nginx/unit"
+LABEL org.opencontainers.image.documentation="https://unit.nginx.org/installation/#docker-images"
+LABEL org.opencontainers.image.vendor="NGINX Docker Maintainers <[email protected]>"
+LABEL org.opencontainers.image.version="%%VERSION%%"
 
 RUN set -ex \
     && savedAptMark="$(apt-mark showmanual)" \