The IP 172.17.0.3 shows you are using a docker bridge network.
This IP is an internal docker address and you can't access it directly.
You need to use the host IP to connect to your web interface.
You said you are using Portainer.
You need to use the same IP, but change the port (http://{raspberry_pi_IP}:{port}/admin).
Example:
Portainer ➔ http://192.168.0.10:9000
Pi-hole web interface ➔ http://192.168.0.10:8080/admin
Replace 192.168.0.10 with your Raspberry Pi IP.
Replace 8080 (the number after the colon) with the port you configured on Portainer (usually one of these values 8080, 80, 81, 8081, 8888).