Build more compact images

[martin-helmich]

Currently, the image is built on top of the regular php images. These do work, but contain a lot of stuff that is not strictly required for a production deployment (like all PHP sources, and a compiler toolchain).

It might be worth investigating if we can slim the resulting image down a bit.

[christianhuth]

Maybe we should work together with crinis, who also develops Docker images for TYPO3 (GitHub, Docker Hub).