Add LABEL to Docker image (#3802)

This label can be used to fetch more information about this image. For example Renovate uses this label to get the changelog of a specific version: https://docs.renovatebot.com/modules/datasource/docker/

docker/dockerfile

@@ -30,6 +30,8 @@ FROM $BASE_IMAGE AS release
 USER node
 WORKDIR /app
 
+LABEL org.opencontainers.image.source="https://github.com/louislam/uptime-kuma"
+
 ENV UPTIME_KUMA_IS_CONTAINER=1
 
 # Copy app files from build layer