Tag Versioning

So as mentioned in #31, I kind of dislike that changes like new extensions etc get pushed into old tags and potentially break old projects or make the version I run on my machine different from the version a co-worker runs on his machine.

So I see a few ways to do this I could think of.

• add the minor php version to the tag and continue like now ~ just don't update old tags. This would result in the cleanest tags and would be consistent with the base php repository chialab/php:7.1.9-fpm. This could potentially break new projects that pin on the newest php release but since they are still new this shouldn't be a problem ~ my concern is about legacy projects.
• add a chialab version. This would make updates kind of complicated since you'd now have to track 2 versions. Also I don't now how to make that look nice.

Start by reading issue #31 and reviewing the repository’s current Docker image tag and release process; the payload names no files or tests. Choose and document one versioning policy, then verify that existing tags remain unchanged and future tags follow the selected scheme.