Docker: Migrate to PHP 7

[raimund-schluessler]

It would be nice if the Docker image could benefit from the increased speed of PHP7.

[Wonderfall]

I agree, we should at least provide a PHP 7 version. There's an official image php:7.0-fpm, and even php:7.0-fpm-alpine so we can finally use Alpine Linux as a base for this image.

[pierreozoux]

Does Nc officially support php 7?

If yes, then, just PR, and I'd accept it, if not, we'd need a tag then.
(You can PR also, and I'd accept it)

[qrevel]

I'm actually using wonderfall/nextcloud and it works really great. I was waiting the official image but I can't switch to it if it doesn't provide php7 using alpine.