It would be useful to tag docker images by their branch name. github.repository can contain incorrect syntax for docker tags. A new context property similar to CI_COMMIT_REF_SLUG (https://docs.gitlab.com/ee/ci/variables/predefined_variables.html) in gitlab would be useful.
Thanks for this feedback! We're always working to improve GitHub and the GitHub Community Forum, and we consider every suggestion we receive. I've logged your feature request in our internal feature request list. Though I can't guarantee anything or share a timeline for this, I can tell you that it's been shared with the appropriate teams for consideration.