Setting reverse proxy from withing docker caddy to localhost service

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Yep, this is correct.
To work around that, you could either add extra_hosts to your docker-compose.yml like so:

      - /appdata/caddy/caddy-data:/data
    environment:
      - EMAIL=user@example.com        # The email address to use for ACME registration.
    networks:
      - proxy_net
+   extra_hosts:
+     - host.docker.internal:host-gateway

networks:
  proxy_net:

Which adds another hostname to /etc/hosts within the caddy container.
With that, you will be able to use that new hostname to reference your actual host docker is running on:

  encode gzip
- reverse_proxy localhost:8000 {
+ reverse_proxy host.docker.internal:8000 {
       header_up X-Real-IP {remote_host}
  }
}

However, you have to make sure your service on the actual host is listening on whatever host.docker.internal resolves to (usually 172.17.0.1 - the docker0 interface).
Meaning, if you configured that service on the host to only listen on 127.0.0.1, you will have to change that.

The alternative would be to run caddy either outside of the container (directly on your host) or with network_mode: host:

        - EMAIL=user@example.com        # The email address to use for ACME registration.
+     network_mode: host
-     networks:
-        - proxy_net
- networks:
-   proxy_net:
-     driver: bridge

The downside here is, that you will have to manually map a port for each Docker service and can’t use your proxy_net. But localhost will refer to the actual hosts’ localhost.

  whoami:
    image: jwilder/whoami
    ## exposes :8000
    ## so we can use `reverse_proxy 127.0.0.1:8000`
    ports:
      - 127.0.0.1:8000:8000
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