Once you fetch the git commit id, pass it on to Docker as build-args via fly deploy: What is the correct workflow for utilising secrets during deployment? - #5 by kurt
Also see: Environment variables - #22 by ignoramous
Note though, don’t use build-args for secrets: How to include `fly-log-shipper` in a project? - #4 by ignoramous
I should have been clearer. FLY_IMAGE_REF is set for a Fly app at runtime (not build-time).