Generation of self-signed certificates

Is your feature request related to a problem? Please describe.
I'm running docker instance in intranet/private-network and require https for some docker-instances (like docker-registry). The private-lan runs on TLD ".lan" so it's not possible to use letsencrypt by routing domain-name over router as letsencrypt only supports domains/tld's reachable from intranet.

Describe the solution you'd like
I noticed nginx-proxy-manager already supports custom-certs which is awesome! I would love to have an additional option in that dropdown in section "SSL Certificates" that could be named "Create self-signed certificate" and then routes this request to f.e. the linux-tool mkcert.

https://blog.filippo.io/mkcert-valid-https-certificates-for-localhost/

mkcert my-private-domain.lan

Describe alternatives you've considered
I could run mkcert on my local machine and manually upload the cert-files into nginx-proxy-manager.

Additional context
I think I'm not the only user who runs a docker instance in private/lan and think this feature would support/help other users as well.

Start by reviewing the SSL Certificates dropdown and the existing custom-certs flow. Trace how certificate requests are handled for proxy hosts, then define how a self-signed certificate would be generated for private .lan domains and made selectable. Done means users can create and use such a certificate without manually uploading certificate files.