Refactor Image in order to drop s2i

Objective is to refactor the image in order to drop s2i base image and decouple apache and wordpress layers. Base work on the mvp branch.

• Change Dockerfile image: ship php-fpm + composer;
• understand what actions from install script are dependent on the db running (those will be on the run-php-fpm.sh) the others on the dockerfile (for now don't separate in 2 docker files like the Drupal)

Work can be based (but simplified!) on Drupal php-fpm layer and images:

• https://gitlab.cern.ch/drupal/paas/cern-drupal-distribution/-/blob/v10.3-0/images/php-fpm/run-php-fpm.sh?ref_type=heads
• https://gitlab.cern.ch/drupal/paas/cern-drupal-distribution/-/blob/v10.3-0/images/Dockerfile-sitebuilder?ref_type=heads
• https://gitlab.cern.ch/drupal/paas/cern-drupal-distribution/-/tree/v10.3-0/images?ref_type=heads

Ignore apache layer.