improve connection between containers

I'm anticipating a lot, just remembering something good I learnt.

We should avoid this kind of waiting:

sleep 15; # We should wait for mariadb container being ready

src https://github.com/guifi/docker-drupal-guifi/blob/master/drupal6/drupal-guifi/drupal-entry.pl#L18

What I suggest is replacing it for something like this:

https://github.com/Bitergia/docker/blob/master/utils/entrypoint-common.sh#L13-L60

This is like a "little framework"-function to easily manage connections to services with adjustable variables:

• timeout of 10 seconds
• tries 60 times (1 time each second sleep 1)

you see that in this repo there are lots of other facilities and useful functions. At the moment I'm not needing the others, but we will see in the future