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 netbirdio/netbird#2242

[CLAassistant]

All committers have signed the CLA.

[benniekiss]

Force-pushed to fix authorship

[iammuho]

When that PR will be merged ?

[johnnypea]

Is this going to be merged anytime soon, please?

[benniekiss]

This PR needs updated to match the correct format for the http2 directive. I'll try to get to that when I can, but I've since moved to a custom dashboard container, so this is not really on my priority list.

[benniekiss]

I've updated the http2 directive and updated the alpine version (it was very very old!). I haven't run any tests since I do not use the regular dashboard container in my deployment anymore, so if a community member wants to test and ensure this works, please do! I believe it should.

@bcmmbaga @mlsmaycon this is ready for review if you are interested in taking a look.