Allow CI jobs using the base image to fail on a base build

This adds a new rule for all CI jobs that require the base image: on Git tags starting with base-, they are allowed to fail. This allows the unit tests to fail, for example, and prevents them from blocking the backend-build-base job which comes at a later stage.

This rule can cause duplicate pipelines in merge requests (one for the commit, one for the merge request itself); GitLab shows a “Job may allow multiple pipelines to run for a single action” warning and recommends using a built-in template to block merge request pipelines entirely (docs).