Change Dockerfile label from legacy to modern notation.

Author: Potherca

Dockerfile

@@ -31,4 +31,4 @@ WORKDIR /app/server/
 EXPOSE 8080
 CMD ["php", "server.php"]
 
-LABEL org.opencontainers.image.source = "https://github.com/pdsinterop/php-solid-pubsub-server"
+LABEL "org.opencontainers.image.source=https://github.com/pdsinterop/php-solid-pubsub-server"