Use Docker layers

[federico-razzoli]

This is just a suggestion. Let's discuss if you disagree.

Building a Dockerfile takes time. We should split the current image into 2 parts: a MariaDB image and a ColumnStore image. In this way, when we modify the CS Dockerfile and build it again, we won't rebuild MariaDB.

Also, probably we can use the official MariaDB image. Or we could make an image that simply installs S3 and changes some configuration (so these steps doesn't have to be done by the ColumnStore image when we make unrelated changes).