fix:src-prometheus add default url

Created by: kevinwojo

See https://github.com/sourcegraph/customer/issues/875 for context.

Customer gets a banner saying that Prometheus is unavailable when alerts are configured on the single docker container server. In other deployments, we explicitly set this, so nobody notices. We also don't have a ton of customer configuring alerts on our single container image as it's really designed for evaluation purposes - a short stop in the Sourcegraph journey.

Test plan

Will run candidate images locally to verify that nothing is broken in the deployment. Also CI should smoke test.