Add support for custom certificate key types via environment variables

[mz2]

Adds support for custom certificate key types through CERT_KEY_TYPE and CERT_ELLIPTIC_CURVE environment variables, optionally also passed to the Dockerfile, to enables ECDSA P-256 certificates and other key types beyond the default.

Maintains backward compatibility when environment variables are empty or not set (i.e. no impact to current behaviour when the vars are not set).

[jc21]

Looks good, could you please add a section to the documentation related to this:

https://github.com/NginxProxyManager/nginx-proxy-manager/blob/develop/docs/src/advanced-config/index.md

So that your changes are not hidden :)

[nginxproxymanagerci[bot]]

Docker Image for build 3 is available on DockerHub:

nginxproxymanager/nginx-proxy-manager-dev:pr-4762

[!NOTE] Ensure you backup your NPM instance before testing this image! Especially if there are database changes. This is a different docker image namespace than the official image.

[!WARNING] Changes and additions to DNS Providers require verification by at least 2 members of the community!