Datadog health check monitors failing

[hackartisan]

The health check is reporting OK but datadog reports failures, and alerts are triggered. See:

• https://app.datadoghq.com/dashboard/96b-77b-bri/dls-health-dashboard?fromUser=false&refresh_mode=sliding&from_ts=1734725663825&to_ts=1734727463825&live=true
• https://app.datadoghq.com/monitors/141404766

The issue appears to be that the checks go to localhost over http, and get a 301 redirect:

$ curl -i localhost/health.json
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=utf-8
Content-Length: 0
Connection: keep-alive
Status: 301 Moved Permanently
Location: https://localhost/health.json
Date: Fri, 20 Dec 2024 20:46:29 GMT
X-Powered-By: Phusion Passenger(R) 6.0.24
Server: nginx/1.18.0 + Phusion Passenger(R) 6.0.24

On other applications' VMs, there's no redirect.

The issue was solved by #6581 but that caused login to fail so it was reverted.

Sudden priority justification

Currently our monitoring is incorrectly triggered so we won't know when an actual problem occurs.