Add docker-compose file for separate containers.

Demonstrate that this pattern works.

This is based on the example at
<https://github.com/fatk/docker-letsencrypt-nginx-proxy-companion-examples/blob/master/docker-compose/v2/simple-site/docker-compose.yml>.

docker-compose-separate-containers.yml

@@ -0,0 +1,33 @@
+version: '2'
+services:
+  nginx:
+    image: nginx
+    container_name: nginx
+    ports:
+      - "80:80"
+    volumes:
+      - /etc/nginx/conf.d
+    networks:
+      - proxy-tier
+
+  dockergen:
+    image: jwilder/docker-gen
+    command: -notify-sighup nginx -watch /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
+    volumes_from:
+      - nginx
+    volumes:
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+      - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl
+    networks:
+      - proxy-tier
+
+  whoami:
+    image: jwilder/whoami
+    environment:
+      - VIRTUAL_HOST=whoami.local
+    networks:
+      - proxy-tier
+
+networks:
+  proxy-tier:
+    driver: bridge