Support for ARM architectures

The attached docker-compose.yml file:

1. Invokes a build of the xwiki image instead of downloading the default. The build works fine on ARM (tested on Rock64 SBC) with no other changes.
2. Using an alpine based MariaDB (10.2.15) image instead of the stock mysql5.7 also seems to work. (xwiki user needs more priviledges to create subwikis)

So far so good.