Flyctl already has a docker image, but the gitlab actions do not seem to contain building and pushing that image to somewhere.
On docker hub, there appears to be an image for that, though: Docker Hub.
Could you either just build and push the image to ghcr.io via github actions or link to the dockerhub one?
Flyctl is build on Github and goreleaser takes care of pushing to Docker Hub. Does this help?
It does, thanks 
Is it documented publicly, that the Docker org belongs to Fly.io?
On docker hub, website links are not verified. Would it be possible to link to these images from a trusted place, so others can trust them without having to search for this thread?