Make the install process more robust

[pcambra]

There hare been a few changes related to #20 to fix the install process.

We should try to make the install a bit more robust, specially this part: https://github.com/contentacms/contenta_docker/commit/db36301db5de52b4895e39c95fab5068b8b08075#diff-85ebb058637f8b5d51ee2d488eb917ca

Maybe it should check whether site is already installed, or some folders exist already, I am not sure if we need to install Drush in the Dockerfile as maybe the local Drush is enough.

Additionally, figuring out how to check if this Dockerfiles build correctly and actually install the project would be a huge win as most of the issues reported here are related to chasing changes in either Drupal, Contenta or the underlying packages and utilities to make the containers work.