Add image source label to dockerfiles

To get changelogs shown with Renovate a docker container has to add the source label described in the OCI Image Format Specification.

For reference: https://github.com/renovatebot/renovate/blob/main/lib/modules/datasource/docker/readme.md

Signed-off-by: Marc <github@wuast24.de>

Dockerfile

@@ -2,6 +2,7 @@ ARG ARCH="amd64"
 ARG OS="linux"
 FROM quay.io/prometheus/busybox-${OS}-${ARCH}:latest
 LABEL maintainer="The Prometheus Authors <prometheus-developers@googlegroups.com>"
+LABEL org.opencontainers.image.source="https://github.com/prometheus/node_exporter"
 
 ARG ARCH="amd64"
 ARG OS="linux"