For compute / builders / registry, yes. For egress, if your app stays within the generous (rationed) 160GB/mo limit. Checkout the pricing page for more info.
It means running the main app process as user-id 0. Don’t worry about it since from the dockerfile you shared, it looks like the process may be running as the default user (which is the root user on Fly). If you’re curious why non-root users do not have access to stdout/stderr, see: Allow non-root users to write to stdout/stderr