Update base images to Debian Stretch and Alpine 3.6

[yosifkit]

Bringing together other issues/PRs requesting base image updates: #428, #456, #457, #468, #482, #501.
Should fix theses once we make a PR: #495, #198

We do not want to maintain our PHP images over multiple distro releases in the long term. To update the base image without immediately breaking users like docker-library/golang#131, we'll have to release images based on all four distributions (Alpine 3.4, Alpine 3.6, Debian Jessie, and Debian Stretch) over a few months and then end of life the versions based on the older Linux distributions (Alpine 3.4 and Debian Jessie).

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Why not keep all distribution versions longer (Alpine 3.4, 3.6, Jessie, and Stretch)?

• it takes over ~1.5 hours for travisCI to test current PRs, this would close to double that (28 uniq tags to ~49)
• it will increase update delay since the build server would also have to build the extra versions
  • this will also increase the likelihood of a tag not available for given architecture since some arches build much faster than others (like rabbitmq:3-management docker image is borked rabbitmq#188)

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We are thinking something like this:

• release all versions and variants of the PHP images on current disto base and newer distro base (we'll use 1st November 2017 in this exploration)
  • so for example, Alpine based images would have tags for php:7.1-alpine3.4 and php:7.1-alpine3.6
  • and the Debian based images would be php:7.1-jessie and php:7.1-stretch
  • current less-specific tags would stay pointing to the older distro (php:7.1-alpine would still be alpine 3.4)
• add note about this process for future base image updates
• place large note on Docker Hub description that images based on older distributions (alpine 3.4 and debian:jessie) will be EOL in 3 months (with an exact date like 1st February 2018)
  • use deprecation notice
  • explain that generic tags like php:7.1-alpine will continue to contain Alpine 3.4 until that date and instead encourage users to switch to php:7.1-alpine3.4 to stick to a specific Linux Distro
  • they will switch over to Alpine 3.6 on that date and the same for jessie/stretch
  • remind users that 7.2 is Alpine 3.6 and stretch only
• on specified EOL date, remove Alpine 3.4 and Debian Jessie based images from the Supported List
  • users can still docker pull, but the images would no longer be rebuilt with base image changes or version bumps

I think @tianon and I could tackle something like this in the next week or two. Any suggestions or comments?

[amq]

Did some unexpected problems arise?

[amq]

https://alpinelinux.org/posts/Alpine-3.7.0-released.html