Create an individual Docker image for the Prometheus Exporter

Currently the prometheus exporter is packaged within the overall Solr Docker image.

While this is useful, it doesn't provide the best experience for users. If used to run the Prometheus Exporter, the Solr docker image contains mostly irrelevant things which add considerable complexity and size to the image.

With a separate image, we could provide a better more intuitive user experience that is actually optimized for running the prometheus exporter, and be considerably slimmer than the Solr image.

Previous discussions:

• SOLR-14915
• docker-solr#307
• docker-solr#224