docker: allow to specify the branch to pull for various repos
Declare an docker build-arg  and use it whenever cloning one of our repositories. If the buildarg is not specified, the variable defaults back to 'master' and thus the current behavior doesn't change.
From the .gitlab-ci.yml file, when building pass the GST_UPSTREAM_BRANCH that's defined from the ci_template as the buildarg so we will be building the corresponding branches for the docker images.
Close #33 (closed)