verify admin behind proxy

[mybigman]

Bug description

Portainer behind proxy in sub directory causes "unable to verify administrator account existence"

Expected behavior
Not to display errors

Steps to reproduce the issue:
Install nginx configured as reverse proxy
Install portainer only exposing port 9000
Configure nginx to point to sub directory

As per https://portainer.readthedocs.io/en/stable/faq.html#how-can-i-configure-my-reverse-proxy-to-serve-portainer

Technical details:

• Portainer version: 1.21.0
• Docker version (managed by Portainer): 18.09.1-ce
• Platform (windows/linux): linux
• Browser: chrome

Additional context
Looks like it was suppose to be resolved here #462

Console also shows -

Error: [$resource:badcfg] Error in resource configuration for action `checkAdminUser`. Expected response to contain an array but got an object (Request: GET api/users/admin/check)

[ghost]

Leaving this to the community to try and resolve, as we do not currently support reverse proxies with Portainer. If you wish, you can join our slack server and the reverse-proxy channel as there seems to be quite a few active users there.