Revert "Explicitly set TAG to latest"

This reverts commit b26ad3fa "Explicitly set TAG to latest"
since it is causing every image that gets built, even the ones on
secondary wip branches, to be tagged as `latest` and thus affecting
production everywhere.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -29,7 +29,7 @@ before_script:
         RELEASE="${CI_COMMIT_BRANCH#*/}"
         TAG=latest
     else
-        TAG="latest"
+        TAG=$CI_COMMIT_REF_SLUG
     fi
     if [ "$STABLE" = "auto" ]
     then