[FEAT] Add Support and Documentation for Varnish 6 and Varnish 7

[damienwebdev]

:bulb: Feature request

Feature Name

Varnish

The Desired Behavior

We need to be able to use Varnish (as opposed to Redis) for FPC caching. I'd prefer this to be opt-in, rather than opt-out, as I've found that local development (what Mage2docker targets) is somewhat convoluted when Varnish is mandatory.

Your Use Case

As a developer, I'd like to use Varnish.

Prior Work

This came up in a discussion with @erfanimani