Back up the server then work through:

Just follow the guide iocaine.madhouse-project.org/…/getting-started/ Bind the iocaine service to 0.0.0.0:42069

cat /opt/iocaine/etc/iocaine/config.d/00-server.txt.kdl 
http-server default {
  bind "0.0.0.0:42069"
  use handler-from=default
}

expose interface to Docker (make sure not to expose it to the world).

sudo iptables -I INPUT 4 \
  -s 172.18.0.0/16 \
  -d 172.18.0.1 \
  -p tcp --dport 42069 \
  -j ACCEPT

add the interface to the proxy service in a custom/docker-compose.yml.template

proxy:
    {{COMPOSE_CADDY_IMAGE}}
    env_file:
      - ./caddy.env
      - {{ CADDY_EXTRA_ENV }}
    volumes:
      - ./caddy/Caddyfile:/etc/caddy/Caddyfile:ro
      - caddy_data:/data
      - caddy_config:/config
    extra_hosts:
      - "host.docker.internal:172.18.0.1"
...

add iocaine to caddy in a custom/Caddyfile.template

{$LEMMY_HOSTNAME} {
        import caddy-common

        ######################################
        ########## log files        ##########
        log cn_blocked {
                output file /data/cn-blocked.log
                format json
                no_hostname
        }

        ########## iocaine  code ##########
        @read method GET HEAD
                reverse_proxy @read host.docker.internal:42069 {
                @fallback status 421
                handle_response @fallback
        }
...

rebuild all the docler images (at least the caddy one ./deploy.sh -f) and test

I think that should work