Support docker unit tests, use child pipeline for parallel builds

• Add support for docker unit tests using https://github.com/GoogleContainerTools/container-structure-test (to fully support these unit tests, we use a specific Gitlab runner providing a docker daemon, running in a VM with a docker executor. This is currently an AWS ec2 in the bastille-dev account, while waiting for a DC VM)
• To better support the unit test feedback of multiple parallel docker builds, use children pipelines triggered by a parent matrix build rather than a loop over the images in a single job.
• Report the Trivy findings as test results
• Report the hadolint findings as codequality on the MR page (but only works for single image builds)
• Add workflow rules to prevent duplicated builds for a branch and its associated MR, or for a commit on master and the associated release tag