Support BYPASS_VARNISH flag to route requests directly to Nginx

[davidalger]

The magento2 environment type (the only one making use of Varnish currently) sends requests to varnish which passes them on to Nginx. There are use-cases where it is desired to send requests directly to Nginx (such as when the Redis backend is used for full page cache) or simply when needing to eliminate Varnish from the mix when debugging tricky issues.

In order to simplify this such that users will no longer be required to setup custom labeling on containers used by the project, a BYPASS_VARNISH flag should be set such that when BYPASS_VARNISH=true or BYPASS_VARNISH=1 is found in the project's .env file Traefik routes to the Varnish container in an environment will be disabled allowing requests to pass to directly to Nginx.