I think this should be merged into a new php7 branch instead of into master.
That way people can choose how to run it.
Updated to use alpine edge and php7.
Added some missing php extensions.
Related to https://bugs.alpinelinux.org/issues/6450
As Alpine 3.5 with PHP 7 is out, it's probably better to use it (once the Docker images are there), see #102.