Fix issue with invalid characters in test image tag name

The CI job for building the docker image currently use the git branch resp. git tag name as image tag. When using characters like '/' in git branch names, the build fails, since '/' is a invalid character for docker image tag names (see e.g. https://git.ufz.de/rdm-software/svm/backend/-/jobs/15857)

This MR should fix this issue by using the predefined CI_COMMIT_REF_SLUG environment variable as image tag name (https://docs.gitlab.com/ee/ci/variables/predefined_variables.html).

Signed-off-by: Norman Ziegner norman.ziegner@ufz.de