Varnish caching is something that isn't a "standard" in the retail hosting sector, so for mass hosting varnish won't be used.
For example; when you have a wordpress website (one of the mainstream website CMS) it needs additional rules to work with varnish.
It could be amazing if enhance gives the option to toggle a varnish caching proxy, you could run varnish in a seperate "custom" container and this way you could at least give 1 or maybe more websites access to varnish. (as long as enhance doesn't need to handle varnish configurations)
Just my thoughts 🙂