fix format for label

Author: ylebre

Dockerfile

@@ -32,4 +32,4 @@ 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"