CI: push 'main' docker image tag on default branch
When running on the default branch, push an image tag called 'main'. This enables the functional testing jobs to reference the image by $CI_COMMIT_REF_SLUG.
This will still push a 'latest' tag on the default branch.
Fixes the registry pull error seen in https://gitlab.com/gitlab-org/distribution/issue-bot/-/jobs/4650419979.
Follow-up to !15 (merged).
Edited by Mitchell Nielsen