gitlab-ci: Tag the Docker image as `latest` when running on master branch
Currently using the generated Docker images requires spelling out their full commit id.
The Docker tools default at looking for the latest tag when not specifying
one, and that's what we need to provide a stable pointer to the most current
image to be deployed.
Test pipeline where the only: master was disabled to check that tagging actually works:
Edited by Emanuele Aina