Better alpine for php7

[kevin-verschaeve]

php 7 image currently use php:7.1-alpine as base image.

Change to alpine, and add php7-phar to apk add

This currently not work and give this error:

fetch http://dl-cdn.alpinelinux.org/alpine/v3.4/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.4/community/x86_64/APKINDEX.tar.gz
ERROR: unsatisfiable constraints:
  php7-phar (missing):
    required by: world[php7-phar]
The command '/bin/sh -c apk add --update php7-phar wget' returned a non-zero code: 1