Not familiar with gitlab CI, but you should ref this GitHub Actions usage with flyctl auth docker: kentcdodds.com/deployment.yml at 61923537967f5f488ca2e9af690163194c09fd98 · kentcdodds/kentcdodds.com · GitHub
Specifically note, username is set to x, docker image tag set to git tip, and deploys done with -i switch linking to a docker/runc image in registry.fly.io.