Allow to install packages in the Nuxeo Docker image at runtime

Following NXP-27514, let's add the ability to pass a NUXEO_PACKAGES environment variable to the image to install some packages at runtime.

This will definitely be useful for at least:

• The Nuxeo Drive team, to be able to run a Nuxeo container with te nuxeo-drive package installed and run the functional tests in a "production-like" environment, without having to rebuild an image.
• Product owners, to test some packages.

In fact, put back what exists for installing packages in the official image.

Note: update the nuxeo/docker README.