prom-wrapper: force use of netdns=go

[jhchabran]

Description

Sister PR of https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/927

NOTE:

Checklist

• [N/A] CHANGELOG.md updated
• [N/A] K8s Upgrade notes updated
• [ ] Sister deploy-sourcegraph-k8s change:
• [x] Sister deploy-sourcegraph-docker change: https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/927
• [x] All images have a valid tag and SHA256 sum
• [ ] I acknowledge that deploy-sourcegraph-k8s is now the preferred Kubernetes deployment repository

Test plan

CI + https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/927