support nginx_* env variables on nginx container and vhosts

[poma]

Implementation of #1324 feature request. Environment variables starting with nginx_* are added as nginx config statements, for example: nginx_client_max_body_size=30M. Variables of jwilder/nginx-proxy container are added to the global config, and variables on others containers are added to the respective vhosts.

Example docker-compose.yml:

version: '2.1'

services:
  nginx:
    image: jwilder/nginx-proxy
    ports:
      - 80:80
      - 443:443
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      nginx_client_max_body_size: 42M

  app:
    image: app
    environment:
      VIRTUAL_HOST: example.com
      nginx_client_max_body_size: 1337M

Currently only lower case variables are supported (until nginx-proxy/docker-gen#306 is merged). Also if there is an easy way to merge Env dicts from all containers with the same vhost, we should do that, currently Env map is taken from the first container for each vhost.

[pnbecker]

I like this PR and think it would be a helpful addition.

[poma]

While we are waiting for this to be merged, you can use updated docker-gen from the repo poma/docker-gen that already contains this PR

[poma]

Rebased PR onto the current master, now also supports upper case "NGINX_*" env vars.

One problem I've encountered is that some containers export export "NGINX_*" env vars that are invalid in the context of nginx conf and this might lead to broken nginx config generation. Notably nginx:alpine exports NGINX_VERSION which is an invalid in nginx config, this might happen if you want to put another nginx instance behind the main nginx proxy. There is a workaround of overriding exported value with an empty string. But it's still a breaking change that can break a working server when people update to it, so be careful.