Dashboard on backup MGRs always redirects to /, breaking reverse proxy support
The backup MGRs always redirect to / on the active MGR. This means if the proxy hits an active MGR for /, the dashboard loads without redirect, but the proxy might try to load other resources such as images from backup MGRs, which then redirect to /. Instead of the requested image, the browser then receives a html page.
If the proxy uses a backup MGR to load $url_prefix/, it redirects to / on the active MGR, so this breaks url_prefixes too.
The change which causes this behavior appeared in 12.2.2.
I'm working on a fix: https://github.com/ceph/ceph/pull/19758