Check also certificate keys using elliptic curve algorithms

[mkochenough]

This should help with issue #663.

[jc21]

Docker Image for build 2 is available on DockerHub as jc21/nginx-proxy-manager:github-pr-665

[mkochenough]

Thanks for this hint. I used this docker image. It still fails to import the key. Unfortunately the error message is empty. I will dig a bit around to fix this.

[MarceloLagos]

The error message is empty because it doesn't fail the openssl command. Instead the openssl command is successful, but the output is empty, therefore it fails later in the check to see if "key ok" is in the result.
Furthermore even if that doesn't fail, the check for "key ok" will fail, since the verification for EC keys doesn't return "key ok" anywhere. It's a different (2 lines) message than the successful check for RSA keys. See my post on #663

[jc21]

Docker Image for build 3 is available on DockerHub as jc21/nginx-proxy-manager:github-pr-665