Thanks @tbay06 and @ahaywood. As mentioned in my thread on Fly the issue here has been identified and a work around is available: RedwoodJS deployment is not working - #5 by rubys - Build debugging - Fly.io
I also see that fix(fastify): Prevent duplicate `@fastify/url-data` registration by Josh-Walker-GM · Pull Request #9794 · redwoodjs/redwood · GitHub has been merged and is just awaiting release which will hopefully fix the default generated start script on the fly side.