Add CI_BUILD_TAG environment variable

it seems that it is currently not possible to determine if we are building a tag or a commit/branch. since GitLab CI know about this stuff (e.g. only: tags in the YAML file) it should be possible to add a CI_BUILD_TAG variable to the environment that is only defined and set the the tag name if a tag is built.

this would be similar to the TRAVIS_TAG variable. (see http://docs.travis-ci.com/user/environment-variables/#Default-Environment-Variables)

Reassigned to @ayufan

Milestone changed to 8.1

mentioned in merge request !1572 (merged)

Status changed to closed

I'm closing the issue in favor of MR !1572 (merged) which resolves the issue.

mentioned in merge request !14785