Issue Using Non-Default Varnish Version

## Description

This patch addresses an issue with building a non-default version of Varnish. The problem arises when the `install-vmod` script is not present in the `/mnt/jrc-confs/varnish` directory, which can lead to failures when building the custom Varnish docker image. This patch ensures that the `install-vmod` script is copied to the correct directory if it does not already exist, allowing for successful builds of non-default Varnish versions.

## Impact

There is absolutely no impact if this patch is applied.

## Changes

- Copy over the `install-vmod` script to the `/mnt/jrc-confs/varnish` directory if it does not already exist.

## If You Accept

You will be able to use non-default Varnish versions without issues.

## If You Reject

Changing the Varnish version may fail leading to the Varnish service not starting correctly.