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
There is no point in doing a different docker image for each build we
are going to do inside an Ubuntu distro. We can later use the same image
for native ubuntu builds, or other cross builds (e.g. android API 21,
etc).
