Automatize the image build and deployment on the registry of release docker images

What does this MR do?

It changes the GitLab CI conf to build release docker images when on a release branch. It tags images according to the branch name, eg: gl-sast:10-5-latest

Why was this MR needed?

It's needed to keep GiLab and the used gl-sast image in sync, without this an unupdated GitLab 10.5 might use a gl-sast image with new 10.6 functionnalities that break the CI jobs.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug

What are the relevant issue numbers?

Closes #17