Compose v3: Moved shared volume to top-level volumes entry

For compatibility for Docker Compose v3, replaced the unnamed volume in the nginx service with a named nginx_conf volume that is shared by both services.

docker-compose-separate-containers.yml

@@ -1,4 +1,4 @@
-version: '2'
+version: '3'
 services:
   nginx:
     image: nginx
@@ -6,18 +6,20 @@ services:
     ports:
       - "80:80"
     volumes:
-      - /etc/nginx/conf.d
+      - nginx_conf:/etc/nginx/conf.d
 
   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
+      - nginx_conf:/etc/nginx/conf.d
 
   whoami:
     image: jwilder/whoami
     environment:
       - VIRTUAL_HOST=whoami.local
+
+volumes:
+  nginx_conf: {}