Add Varnish module collection to container

[kmcrawford]

Varnish module collection of modules ("vmods") extends Varnish VCL used for describing HTTP request/response policies with additional capabilities. This feature request will extend the functionality of varnish to the kube-httpcache repo.

More info: https://github.com/varnish/varnish-modules/blob/master/README.md

Example of installing VMODS: https://github.com/varnish/varnish-modules/blob/master/Dockerfile