Docker images overhaul

[fabiang]

Hi,

Imho the Docker files need an complete overhaul. There are a few things I've noticed:

1. The Alpine FPM image is missing on Docker Hub.
2. Would be awesome to have generic tags for each major and minor version, e.g. 6-alpine, 6.0-alpine, 6.1-alpine.
3. The Apache images should have the Alpine FPM image as the base image from where to copy the source of Snipe IT. Since this is PHP which works on any distribution it doesn't matter that you copy into a Ubuntu image from Alpine. This would reduce the code needed to create all images, since your only have one file where Snipe-IT is correctly installed.
4. There are many artifacts in the images that make the images bigger, e.g mysql-client, Composer's cache, git, many source files in the FPM package that aren't required for production.
5. There are two entrypoint files, which function different depending on the image. For example the Alpine images doen't support loading some environment variables from file, but the Ubuntu image does.
6. All images install a different set of PHP extensions and also configure them different.

I would start working on these, but first I wanna start a discussion before investing few hours into it.