docker: allow to specify the branch to pull for various repos

docker: allow to specify the branch to pull for various repos

Declare an docker build-arg [1] 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

[1] https://docs.docker.com/engine/reference/builder/#arg