Allow NPM to upload custom certs from a Docker volume defined in the compose file

Is your feature request related to a problem? Please describe.

My NAS has its own SSL certificate which auto-renews every few months. I prefer to use the same cert in NPM, so after each renewal I import it as a custom cert to use with my proxy hosts.

Problem: Since custom certs must be uploaded through the web portal, it's a pain to export my NAS cert files to my PC, then upload them to NPM. This is also a bit of a security concern.

Describe the solution you'd like

Make it so that NPM can use a volume path defined in its Docker compose file to import cert files from, in addition to web uploads. I could then simply mirror my NAS cert to that volume path and forgo the export altogether.

services:
  app:
    image: 'jc21/nginx-proxy-manager'

 [ ... ]

volumes:
      - path_to_my_server_cert/cer_and_key_files:/cert_import_folder:ro

[ ... ]

Then, in NPM, have the option to select /cert_import_folder as the location to upload a custom certificate from.

Describe alternatives you've considered

Can't really think of any alternatives.

Additional context

To take this a step further, perhaps NPM could monitor the cert import folder for any changes (to the NAS cert) and automatically update the custom cert (within NPM) without having to create a new one, switching proxy hosts to the new one, then deleting the old expired custom cert. But that's for another day...

The issue does not identify repository files or tests. Start by tracing the custom certificate upload flow and how Docker Compose volumes are exposed to NPM. Done should mean a mounted certificate directory can be selected as an import source alongside web uploads, without requiring the later automatic-renewal monitoring idea.