The Dockerfiles with scripts are copied from the original repo at
https://github.com/docker-library/php, at commit
781d1cf992df221bb4e5b46f870ad4b50d637387, with these changes:
Base off Alpine 3.5 instead of 3.4, which gives us access to new packages
(for us, specifically the Tidy packages) which we need in child images.
Change from OpenSSL to LibreSSL, as the new default in Alpine 3.5.