Issue Getting 503 backend fetch filed using varnish docker

[hamza-24]

Server operating system version

Centos 7

Plesk version and microupdate number

18.0.56#2

Hey everyone.
I have configured docker with varnish image. When i connect it to my domain using docker proxy it gives backend fetch failed error.
I have followed all the steps given by plesk.
Please help me out in this regard