expose solitude in nginx

allow urls prefixed with solitude, or not

r? @muffinresearch 
This is a minor nit and we could definitely leave this to later. 

This env var is being used to trigger something that's dev-environment specific. If we want to use the docker images for other environments in the future we might want to leave out dev-env specific cases - so it would be nice to make the switch more directly associated with what it does rather than being abstract (My fault for introducing the IS_DOCKER env var) and then move it to fig.yml rather than have it directly in the Dockerfile.

I've filed this bug so we can come back to it later and clean this up across the board. https://bugzilla.mozilla.org/show_bug.cgi?id=1097545