Initial Configuration: Grafana and Prometheus inside docker and then AWS
Okay, it took time and help from someone else. I was going down the path of installing nginx, but that won't end up being necessary.

apiVersion: 1

datasources:
  - name: Prometheus
    type: prometheus
    # &lt;string, required&gt; access mode. proxy or direct (Server or Browser in the UI). Required
    access: proxy
    orgId: 1
    url: http://localhost:9090
    isDefault: true


This is the end change. originally I had access: direct. As I understand it, that means access to the metrics is via the browser. Switching it to proxy means that Grafana will look up the data. Which means the URL doesn't need to be externally-useful.

To a newbie, none of this was obvious, so maybe my two days of pain will help someone out in the future.