Updated Alpine version to 3.6 for PHP 7.1

[elegos]

This pull request updates the Alpine version to 3.6 for PHP 7.1 images.

[tianon]

See also #357, #360, and docker-library/golang#131 (comment) for where a similar change was made previously, where it was reverted, and the discussion that led to the revert. 😞

[elegos]

@tianon: uhm, maybe we could create a new tag? I mean, I need Alpine >= 3.5 due to new packages in the repositories (in particular: shadow). I can of course continue using giacomofurlan/php (my public docker hub repo), but it'd be a pity not to include it in the main one.

Thanks :)

[greut]

golang creates images for alpine 3.4, 3.5 and 3.6. https://hub.docker.com/_/golang/

[soullivaneuh]

golang creates images for alpine 3.4, 3.5 and 3.6. https://hub.docker.com/_/golang/

This is a good alternative solution.

Also, this should be dispatched to 7.0 too, and maybe 5.6.

[tianon]

The thing that makes that solution more palpable for "golang" is that "golang" only has two supported versions, and for each supported version, we only support "jessie"/"stretch" and "alpine3.4"/"alpine3.6" variants, so the full list of uniq supported tags (and thus Dockerfiles) is only 11, even with added Alpine variants. In the case of PHP, that number is _currently_ 28. There are four supported versions of PHP, and each version has at least seven supported variants/configurations. Additionally, "golang" builds relatively quickly while "php" does not.

[tianon]

Additionally, even if we only do this for a single version (say, 7.1), it will add at least 3 new variants (since we support php:7.1-alpine, php:7.1-fpm-alpine, and php:7.1-zts-alpine).

[soullivaneuh]

In the case of PHP, that number is currently 28. There are four
supported versions of PHP, and each version has at least seven supported
variants/configurations.

Maybe time for scripting a little bit more? 😄 Example: https://github.com/nexylan/docker-php-dev/blob/36ccdfc5ddabd3e2074a7bee43c3dc8bf399e64d/update.sh

Additionally, "golang" builds relatively quickly
while "php" does not.

Well, maybe this can be improved but it's not a big problem IMHO.

[yosifkit]

Closing in favor of discussion to create base distro update process #504.