DOCKER: create intermediate image having "composer install" done

Speed up build process by creating an intermediate image containing the composer install step.

A "child" image could use this intermediate one, as long as there is no modification to composer.json.

The composer install could be made by checking out the repos on the master branch ; this way the "composer install" step is done.

The "child" image would just have to copy the source code into the container (copy the source code or volume-mount it)