Oh this will work really well for Gatsby: https://fly.io/blog/serve-small-with-fly-io-and-gostatic/
They have a build image you can use for this, I think if you do this Dockerfile it might just work for any ol’ Gatsby project:
FROM gatsbyjs/gatsby:onbuild as build
FROM pierrezemb/gostatic
COPY --from=build /app/public/* /srv/http/