Looks like you got some help over in the FusionAuth containers project.
I'm not super familiar with caddy, but you can also see other reverse proxy configurations (including which headers they forwarded) here: https://github.com/FusionAuth/fusionauth-contrib/tree/master/Reverse Proxy Configurations
If you get it figured out, please consider submitting a PR to that repo with the caddy config so that others can benefit from your findings.