Add intermediate docker containers: Speed up builds

[christianlupus]

Currently, only one caching container is used for all CI builds. Due to the matrix quite some work is done during the building. There is the suggestion to use multiple cache stages to enhance this:

The first stage consists only of the PHP container with all dependencies installed/built. That should be 3 containers for the different PHP versions.

Secondly, for each required combination of (PHP version, NC version, DB type) a container is generated that bases on the 1st stage cached ones and contains the NC server data (including submodule) and a DB dump to be used for plain installation as well as the vanilla data folder. An appropriate entrypoint will initialize the DB according to an env variable before the DB can be used. The restoration can be sped up by disabling temporarily all security measurements as no concurrent access is possible.