Add alpine v3.6 and alpine v3.7 as build targets for php 7.1

[clingle]

Due to some composer dependencies not yet having support for php 7.2, some people may want a newer version of alpine for running php 7.1 for various reasons. This is also in light of alpine v3.4 reaching end of support in May 2018

My particular use case was the necessity for the "tidy" module for PHP which requires libtidy.so, which is only available in the community repository in newer versions of alpine.

[tianon]

Every additional variant adds a significant load of additional build time, especially when talking about variants like these where each extra Alpine version translates to no less than three additional variants where we have to do a full build of PHP.

Given this and the fact that Alpine 3.6 doesn't receive a lot of maintenance after the 3.7 release, we're not likely to accept additional variants for Alpine versions older than the latest release.

[clingle]

That makes sense, thanks for your response. I just proposed Alpine 3.6 as well because PHP 7.2 images had support for it. Is there any interest (other than my own) in still adding an Alpine 3.7 variant of PHP 7.1?

[tianon]

Yeah, I think Alpine 3.7 is something we ought to look at doing (ie, finally actually resolving #504).