Ensure docker-php-ext-* are executable

Summary:
docker-php-ext-configure and docker-php-ext-install weren't executable.

This has been been discovered at T641#5836.

Test Plan: Build a container, then docker-php-ext-install some-extension.

Reviewers: dereckson

Maniphest Tasks: T642

Differential Revision: http://devcentral.nasqueron.org/D146