Create a staging build for Docker image

Create a staging folder that would be manually updated in order to be able to create staging Docker images.

Staging is manually declared in DockerHub to produce a nuxeo/nuxeo:X.X-staging image.

This allows to run the staging zip in a Docker environment:

• test that the Docker image doesn't miss something (for instance new mandatory params)
• anticipate any problem related to Docker

However, the final (nuxeo:X.X) official image will be rebuilt after release meaning that it may differ if the upstream distribution packages have changed in between.