Run sudo docker build -t "caddy-dns:v#" ., you should see it in the list when you run sudo docker images. Note even though the documentation kinda states that you don't need to open ports if using DNS challenge, but you do when reverse proxying.

To use the caddy-dns/porkbun in the Caddyfile you'll need to define where it's used. Note I'm using environment variables, but see that I'm mapping from subdomain.domain to domain:port.

# globally
{ # Global Options
  email {$LETS_ENCRYPT_EMAL}
  acme_dns porkbun {
    api_key {$PORKBUN_API_KEY}
    api_secret_key {$PORKBUN_API_SECRET_KEY}
  }
}

pairdrop.{$DOMAIN} {
  reverse_proxy {$DOMAIN}:3100
}

To reload Caddy after making changes to your Caddyfile:

sudo docker compose exec -w /etc/caddy caddy caddy reload

References

• https://caddy.community/t/docker-build-with-modules/19255