Support custom SSL certificates for the dashboard

[benniekiss]

This PR refactors the init_cert.sh script in the docker container to support using custom SSL certificates.

I've added two env-vars, NGINX_SSL_CERT and NGINX_SSL_KEY, which when set, will configure the nginx default.conf with the provided port and certificate paths. When LETSENCRYPT_DOMAIN is set, NGINX_SSL_CERT and NGINX_SSL_KEY are ignored.

I've also made it so that the dashboard port can be changed without providing certificates. This way, setting NGINX_SSL_PORT will change the nginx listen port. It may be worth considering changing the env-var name.

This partially closes https://github.com/netbirdio/netbird/issues/2242