Only support mainline/alpine for LibreSSL.

Modify the Dockerfile to fetch LibreSSL 2.5.0 and build nginx against
it instead of OpenSSL.

From https://github.com/nginxinc/docker-nginx

Conflicts:
	mainline/alpine/Dockerfile
	mainline/jessie/Dockerfile

From https://github.com/nginxinc/docker-nginx

Conflicts:
	mainline/alpine/Dockerfile
	mainline/jessie/Dockerfile
	stable/alpine/Dockerfile
	stable/jessie/Dockerfile

Now that Alpine 3.5 is released, and an official Docker image exists for
it, lets use that and avoid building our own LibreSSL in the image.

Restores the stable branch for Alpine and lets that use 3.5 as well.