Refactor project structure

[fquffio]

As the number of PHP versions increases, the number of Dockerfiles raises in a linear way. This brings the repository in a state that is hard to maintain: adding a feature requires a lot of files to be modified in the same fashion, there is a lot of duplicated code, we lack a single source of truth, and so on.

We could refactor repository structure in a way that takes advantage of Docker build arguments to have a single Dockerfile that is used as a base for all the images.

In order for automatic builds to keep working, it will probably become required to have a custom build hook.