Run functional tests against the current SHA

Runs functional tests against the current SHA. This ensures that, whether on master or a feature branch, the functional_tests job will run against the relevant change rather than the default tag of latest.

To accomplish this, we add another Docker build job to the release stage that will push an image with the tag $CI_COMMIT_SHORT_SHA. This job will always run, regardless of branch, so that the functional tests have a valid tag to pull whether on master or a feature branch.

Closes #68 (closed) Related to https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/issues/80