gpg: keyserver receive failed: Address not available

When building the image, that error is reached and building fails.

Solution: change line [#89](https://github.com/elisiano/docker-nginx-modsecurity/blob/master/Dockerfile#L89) from `&& gpg --keyserver ha.pool.sks-keyservers.net --recv-keys "$GPG_KEYS" \` to `&& gpg --keyserver hkp://pool.sks-keyservers.net --recv-keys "$GPG_KEYS" \`.