Add master support for SAML env

This consist in 2 parts:

1. write an on-the-fly image for nuxeo retrieving the latest master build
   • Target repository:
     https://github.com/nuxeo/nuxeo-tools-support-priv/tree/master
   • Target folder:
     docker/images/postgresql
   • The archive and the checksum filenames can be retrieved using:

     curl -s https://qa.nuxeo.org/jenkins/job/Deploy/job/IT-nuxeo-master-build/lastSuccessfulBuild/api/json| jq -r .artifacts[].fileName | grep ^nuxeo-server.*tomcat.zip$
     curl -s https://qa.nuxeo.org/jenkins/job/Deploy/job/IT-nuxeo-master-build/lastSuccessfulBuild/api/json| jq -r .artifacts[].fileName | grep ^nuxeo-server.*tomcat.zip.sha256$
     

   • then the download link can be built using:

     https://qa.nuxeo.org/jenkins/job/Deploy/job/IT-nuxeo-master-build/lastSuccessfulBuild/artifact/archives/<FILENAME>
     

2. write a docker-compose file that will use this master image

This could be based on the current latest image:
https://github.com/nuxeo/docker-nuxeo/tree/master/10.2

And the master one:
https://github.com/nuxeo/docker-nuxeo/tree/master/master

This may be called nightly instead of master to avoid any confusion with the official master image.

Add NUXEO_DEV_MODE variable to the compose and the vars script to make this easily available.