I cant use another backends except eeacms/hello

[AzamAhmadloo]

Hi, I'm running the following docker-compose file but the service has error, "No backends or directors found in VCL program, at least one is necessary."
I changed backend images and just eeacms/hello worked.
How can I use other images as varnish backend?

version: "2"
services:
web1:
image: alpine
environment:
PORT: "7000"
ports:
- 7000
web2:
image: python:3.5
ports:
- 7000
restart: on-failure
varnish:
image: eeacms/varnish
ports:
- "80:6081"
- "6085:6085"
depends_on:
- web1
- web2
environment:
BACKENDS: "web1 web2"
BACKENDS_PORT: "7000"
DNS_ENABLED: "true"
BACKENDS_PROBE_INTERVAL: "3s"
BACKENDS_PROBE_TIMEOUT: "1s"
BACKENDS_PROBE_WINDOW: "3"
BACKENDS_PROBE_THRESHOLD: "2"
DASHBOARD_USER: "admin"
DASHBOARD_PASSWORD: "admin"
DASHBOARD_SERVERS: "varnish"
DASHBOARD_DNS_ENABLED: "true"